“These violent delights have violent ends, and in their triumph die like fire and powder, which as they kiss consume.”

• “…Fire and powder…”, when fire and gunpowder touch they explode and are used up.
• “…in their triumph die…”, in the win of their marriage, they wont get to celebrate this win, because they will blow up.

So I think that the meaning behind Friar Lawrence’s quote is that; their marriage will be a win but will be not long celebrated and will end like fire and gunpowder (quickly used).