Answer:
n Act I, scene iv of "Romeo and Juliet", Romeo is about to tell Mercutio about a dream that he had. Mercutio is quick to cut Romeo off and tell him about Queen Mab. According to Mercutio and the speech that he gives in this scene, Queen mab is the dream fairy -- she is the fairy who gives people their dreams and decides what they will dream about. There are several examples that are given within this speech -- for example women dream of kisses, priests dream of tithes, lawyers dream of money (fees), etc. Within this speech (toward the beginning) Mercutio also discusses how small this Queen Mab is.