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.