In Romeo and Juliet, Queen Mab is the fairy's midwife. As Romeo, Mercutio, and Benvolio walk to the Capulet's masquerade party, Mercutio weaves a long tale about this fanciful, mythic figure. (You can learn more about her in Act I, scene iv.)
Mercutio talks about Mab because Romeo's asserts that he had a dream and that dreams tell truths. Mercutio responds that dreams lie and that Queen Mab uses them to spread mischief.
Queen Mab is so tiny that her wagon is driven by a gnat, and the spokes in the wagon wheels are made of spider's legs. She drives her wagon across people's faces as they sleep. She leads various people to dream of the base desire of their hearts. Mercutio paints her as a distasteful figure.
Romeo interrupts Mercutio to tell him he talks of nothing. Mercutio responds that is what dreams are: nothing. Nevertheless, Romeo has a feeling of foreboding as they near the party.