Good question - let's go through this little speech from Act 2, Scene 6, line by line and find out what the Friar is trying to say to Romeo:
These violent delights have violent ends
And in their triumph die, like fire and powder,
Which, as they kiss, consume.
So - things that you really violently want ("violent delights") can only finish up ending violently. And, as they finally come together - that is, the "violent delights" of Romeo and Juliet - what will happen is they will die: like when a flame and gunpowder come together, as the two things "kiss", they "consume" each other and die. Explosive stuff!
The sweetest honey
Is loathsome in his own deliciousness
And in the taste confounds the appetite.
Even the sweetest honey can be too sweet: you can have too much of a good thing. The sweetness becomes horrible by being too sweet: and when you taste it, it makes you not want to taste any more.
It's a bit like saying heating food up to 50 degrees will cook it, but heating it to 100 degrees will burn it. It's better then, to work at a slightly lower temperature:
Therefore love moderately: long love doth so;
Too swift arrives as tardy as too slow.
Love moderately is the Friar's advice: and advice that Romeo never takes. "Long love" goes slowly - because going too fast, ("too swift") in the end means that you arrive as late ("tardy") as the person going "too slow".
Hope it helps!