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.

Translation

He is talking about dreams about how they function and dreams are nothing but your imagination like thin air and is hard to predict than wind which can blow and get angry and blows south. The wind is blowing them off there course.

“True, I talk of dreams,
Which are the children of an idle brain,
Begot of nothing but vain fantasy”
Shakespeare uses this metaphor to compare dreams and saying that they only lead you in the wrong direction. This is also personification because children cannot come from the brain. Mercutio talks about love being just a dream and you should try not to follow this path and not take the risk to someone who your not guaranteed to be with for the rest of your life. Also Mercutio talks about the wind “woos” this might mean that the wind can change at any time and anything can happen.