Friar: 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. (II.vi.9-15)
Friar Lawrence is telling Romeo to be moderate in his love. He uses the analogies of fire and gunpowder and the taste of honey to explain how the excess of anything is unhealthy and dangerous. He says that fire and gunpowder (powder) turn into fire when they touch (kiss). He is implying that an excess of emotion -- namely an infatuation or obsessive love -- can be combustible and lead to disaster. This is a neat bit of foreshadowing, as the example of gunpowder is a grim one, implying future destruction.
The example of the taste of honey becoming "loathsome in its own deliciousness" is a somewhat more orthodox example of love. The sensual pleasures, if too deeply, or "violently" indulged in (such as the pleasure of love) can, in time, become distasteful to the indulger. Also, the excess of sweets can cause the lack of appetite for other things; this analogy would liken the excess of love to a decreased love of life. No doubt this is a direful warning, and is in Romeo's case particularly apt. At the end of the play Romeo no longer wants to go on living, because he believes that his love, Juliet, is dead.
Then the Friar counsels that Romeo "love moderately: long love doth so". If Romeo loves Juliet moderately, rather than extremely or violently, the Friar is saying, there is a better chance of their love being a long love. This is fairly typical advice someone would give a young bridegroom on the day of his wedding. "Too swift arrives as tardy as too slow" means that it is just as disastrous to love too much too soon as it is to love not enough. In either case the result is the same: the destruction of the love. Friar Lawrence is worried that Romeo loves too much, and that this will cause a sad end for the lovers. This is all meant to foreshadow the tragic end of the play.