In shakespeare's Romeo And Juliet, what does Mercutio's Queen Mab speech mean? Does it teach a lesson or something?

Romeo claims that dreams can be real or true because they reflect reality, while Mercutio argues that they lie. His speech goes on to claim that they reflect the individual's desires and passions. For example, when Queen Mab visits lovers, they dream of love, lawyers dream of money, and courtiers dream of curtsies