MERCUTIO

True, I talk of dreams,Which are the children of an idle brain,Begot of nothing but vain fantasy,Which is as thin of substance as the air,And more inconstant than the wind, who woos,Even now the frozen bosom of the north,And, being angered, puffs away from thence,Turning his face to the dew-dropping south

MERCUTIO
True. I’m talking about dreams, which are the products of a brain that’s doing nothing. Dreams are nothing but silly imagination, as thin as air, and less predictable than the wind, which sometimes blows on the frozen north and then gets angry and blows south.

My opinion/what is the metaphor:

I think the whole quote by itself is a metaphor because it is comparing dreams to being childish and fake and that sometimes trick your thinking into something else.

Now Shakespeare uses this metaphor (I think) to compare dreams to being fake and saying that the only thing they do is mislead you into a wrong path so I would think that he is trying to talk about love is just a dream that takes you in wrong direction so you should also try to follow not just your hearth but also your brain and say to yourself is she really worth it to risk, everything and be depressed that badly when you don’t know if you are actually going to be with her for the rest of your life(which is not actually the truth since he meets Juliet).That is what I think of the metaphor and what is the actual meaning behind the metaphor.