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.
Romeo and his companions are on their way to attend the banquet in Capulet's house. Romeo, at present, is very unhappy and upset because Rosaline, his infatuation, has decided to be a celibate for the rest of her life. He has agreed to go to the banquet in his enemy's house because Rosaline has also been invited and he hopes to meet her there.
But now as they near Capulet's house Romeo says it is not wise to attend the banquet because he has dreamt a bad dream: "I dream'd a dream to-night." Immediately even before Romeo tells him what his dream was, Mercutio dismisses contemptuously his fears as the work of Queen Mab and therefore imaginary.
The speech is important for the following reasons:
1. ATMOSPHERE: Shakespeare's plays were staged in the afternoon. But this scene takes place in the night and the long poetical description about Queen Mab appearing in everyone's dream in the night would have created the necessary impression in the theatre that it was night time.
2. MYTHOLOGICAL INTEREST: Shakespeare's contemporary audience would have readily understood and appreciated the nocturnal activities of Queen Mab the queen of fairies; and it would have created a sense of fear and awe in their minds.
3. THE NATURE OF DREAMS: are dreams merely a figment of a person's imagination as Mercutio affirms or do they provide an insight into our subconscious and foretell our future as Romeo surmises?