These lines are spoken by Friar Laurence to Romeo, in act 2, scene 6, just before Romeo and Juliet get married. Friar Laurence is warning Romeo not to be too rash, reckless, or extreme in the way that he loves Juliet.

When he refers to "these violent delights," Friar Laurence is referring to the violent intensity with which Romeo and Juliet love one another. He warns Romeo that unless he and Juliet learn to love one another more moderately, their love shall result in "violent ends." Later in the play, we discover that this warning was prescient, as Romeo and Juliet's love does indeed end in violence.

Friar Laurence also compares Romeo and Juliet's love to "fire and powder." The image evoked here is of a trail of gunpowder which, when set alight, becomes a trail of fire. Often there is an explosion at the end of a gunpowder trail, and in this sense this image too foreshadows Romeo and Juliet's fate. Their love is like a trail of gunpowder running throughout the play which leads, inevitably, to an explosion. Just a few years after the play was first performed, a group of Catholics tried to blow up the House of Lords with barrels of gunpowder. The allusion in the play to "fire and powder" would have thus had a particularly strong resonance for audiences watching the play at this time.

Continuing with the same theme of loving recklessly, Friar Laurence then compares Romeo and Juliet's love to "the sweetest honey," which, when eaten too greedily and too quickly, becomes "loathsome" and can make one feel ill. In other words, Friar Laurence is saying that Romeo and Juliet are feeding upon their love too greedily and too quickly, so it will make them ill and the love "loathsome."

After comparing their love to a trail of gunpowder and too much sweet honey, Friar Laurence tells Romeo to "love moderately"—preserve the love rather than destroy it. Romeo, of course, is too naive and too much overwhelmed by his first experience of reciprocated love to heed Frair Laurence's advice. He continues to love violently and greedily, and his love, accordingly, ends in violence.

First, Shakespeare uses an oxymoron here: "violent delights." By doing so, he highlights the relationship of the young lovers. While they delight in each other, their love is based in the violent hatred between their families. While they delight in each other, Romeo will commit an act of violence against Tybalt, which will end their delight together. Their relationship is wrapped in duality, borne in intense passions of both love and hatred.

These lines also foreshadow the ending of the play. Romeo and Juliet will meet "violent ends" and will die on the same night, with Juliet using Romeo's dagger to commit suicide and die with her new husband. Rome tries to consume Juliet's poison with a kiss and resorts to consuming his own poison.

The friar warns the couple to "love moderately," as that will bring them "long love." Of course, moderation is not written in the stars for this young couple, who wed within twenty-four hours of meeting each other. Their intense passions and inability to moderate their emotions will end their love and lives quickly.

Friar Lawrence introduces the motif of violent love and reaffirms the theme of age versus youth with these words.

• Violent Love

In Act II, Scene 6, the concerned Friar Lawrence hopes that the future will not punish the young lovers with sorrow, and he warns Romeo against his haste in love. He urges Romeo to act in moderation, for that is the key to lasting love and happiness. Impetuousness always is dangerous since a person discards any caution, he tells Romeo. This scene also foreshadows Romeo's haste in interfering with the quarrel of Tybalt and Mercutio in Act III, in which violence is connected to love.

• The Theme of Age Versus Youth

This passage also illustrates the theme of the wisdom of age against the impetuousness of youth. The wiser and older Friar Lawrence understands the dangers of the two young lovers' marriage in light of the feud between their families. The Friar is worried about the possible repercussions of this marriage.

These words are uttered by Friar Lawrence just before he performs Romeo and Juliet's marriage ceremony.  References to honey and gunpowder may seem strange for a pre-wedding speech, but the crux of what Friar Lawrence is saying is that Romeo and Juliet need to slow things down a bit.  Like gunpowder and a spark coming together, things that are brought hastily together can sometimes explode.  Friar Lawrence also delivers these lines to help Romeo and Juliet to understand that, in truth, what they are feeling at this point is more lust than love.  If they are to build a long-lasting marriage, they will need to love more "moderately" and pace themselves.  As he says, "Long love doth so."

What this illustrates is that, while the friar has consented to marry the two, he feels the need to warn them that their passion will dissipate as the marriage progresses and that they will need to build their marriage's foundation on more stable footing.  That is the key to a lasting marriage.  Sadly for Romeo and Juliet, they will never have a chance to put Friar Lawrence's advice into action.

Both Romeo and Juliet are in a frantic rush to get to the altar after only having known each other for one day. In this scene, Romeo is on fire for Juliet to arrive at Friar Laurence's cell so that they can get married. Friar Laurence counsels Romeo to moderate his passion. He tells Romeo that if he is too on fire, if his love is too passionate ("violent") it will come to a bad end. He compares it to fire meeting gunpowder, which explode ("consume") when they touch ("kiss"). He says, essentially, that if Romeo isn't careful, his love will burn out far too quickly. Friar Laurence then compares too much love to too much honey. If you eat too much of it too fast,  he says, it may taste sweet at first, but you quickly get filled up and then get sick of it and lose your appetite for it. He is telling Romeo to slow down on the passion. A more moderate love will last longer, he says, and a love that arrives too quickly is as bad as one that comes too late.

Saying "violent delights have violent ends" also foreshadows what is to come for these lovers who will soon commit suicide.

These words are spoken by friar Laurence in response to Romeo's request that he should hurry up to conduct his and Juliet's marriage. The words are actually advisory in nature and the friar uses metaphors to allude to Romeo and Juliet's rushed conjugation.

Friar Laurence is saying that the couple's fiery and aggressive passion would end just as savagely as it had begun, implying that this great desire for each other would suddenly die at its pinnacle, just as fire and gunpowder do. The one ignites the other, and the burst they create exists for a brief but exhilaratingly profound period and then quickly fizzles out. 

Friar Laurence uses another comparison when he further states that honey which is very sweet ironically becomes abhorrent because it contains too much sweetness. Such honey is distasteful and when one has a taste of it, one is put off. Because of its gross sweetness, the honey's true value cannot be appreciated and its taste cannot be truly savored.

The friar advises Romeo to love in moderation since this is the quality of an enduring love. If he rushes into love, he might never achieve true, meaningful and lasting love (more haste, less speed). It would only be a short, bright flame that would quickly die out. 

I think we need to see this important speech from Friar Lawrence as a caution to Romeo. Note how Friar Lawrence has seen Romeo overwhelmed by his sudden love for Juliet. He has cast aside his romance with Rosaline and now speaks using intense language to describe his affections for Juliet. Note what Romeo says immediately before this speech:

Do thou but close our hands with holy words,

Then love-devouring death do what he dare--

It is enough I may but call her mine.

Of course, there is irony in this speech, as "love-devouring death" does do what he "dares," and Romeo finds it is not enough to just be able to call Juliet his. Thus it is that the Friar begins by cautioning that "violent delights have violent ends." There is a sense in which Friar Lawrence foreshadows the ending of Romeo's relationship with Juliet. He tries to counsel Romeo to not be in so much of a hurry and to "love moderately" because love that lasts a long time does this; it does not consume itself as fire or power.

Here's a rough paraphrase of Friar Laurence's lines, and I've tried to keep it in the same line configuration as Shakespeare does so you can see which line refers to which:

Violently-begun affections end violently,
And, as they come to fruition, they die. Just like fire and gunpowder,
Which, as you put them together ("kiss") blow up. The sweetest honey
Can be sickly in being too sweet,
And tasting it can make you not want to eat it.
Therefore, Romeo, love moderately: long-lasting loves do that.
Too quick, in the end, comes as late as too slow does.

These violent delights have violent ends
And in their triumph die, like fire and powder,
Which as they kiss consume: the sweetest honey
Is loathsome in his own deliciousness
And in the taste confounds the appetite:
Therefore love moderately; long love doth so;
Too swift arrives as tardy as too slow.

Hope it helps!