The Queen Mab speech contains some of Shakespeare's most biting criticism of his society, and it informs many of the events in Romeo and Juliet.
The speech begins with Mercutio painting the picture of a seemingly benign fairy: "She comes/In shape no bigger than an agate-stone". But his subject quickly loses her innocent appearance as she is associated with infection: "Not so big as a round little worm/Prick'd from the lazy finger of a maid".
Mercutio's rant begins to pick up speed and he flings insults about lovers, courtiers, lawyers, ladies -- all dreaming of individual, personal, materialistic reward. He targets the clergy, accusing them of using their professions as means to a selfish ends.
Finally, Mercutio taps into Shakespeare's most recurrent theme: the interaction of Eros (love) and Thanatos (death) -- obviously a focus for the action in Romeo and Juliet. Mercutio questions the motives of the brave soldier, who appears to be fighting for altruistic reasons, but secretly loves the physical act of battle and murder. In the next breath, he calls to mind the bestial side of sexuality, focusing not on the loving union of souls but the painful and bloody aspect of childbirth.
Romeo tries to soothe his friend, but it is Romeo's insincerity about love that has ignited Mercutio's rage.