Romeo, Benvolio, and Mercutio are on their way to the Capulets party while Romeo and Mercutio discuss whether dreams represent the truth/ inevibility of fate
"In bed asleep while they do dream things true"(1.4.57)
"I fear too early, for my mind misgives...Of a despised life closed in my breast by some vile forfeit of untimely death(1.4113-1.4.118)
In this scene, it portrays two different ideas of dreams. Mercutio thinks that dreams lie as they are based on people's desires. He launches into the story of Queen Mab, a mystical being who visits people to give them dreams based on what they desire. For example a lover dreams of love, a lawyer dreams of money, and a courtier dreams of curtsies
But on the other hand, Romeo believes that dream can be true or even real as they tend to represent reality. His dream was that something bad will happen at the Capulets party and it will result in an untimely death. And, as the audience already knows, Romeo and Juliett commit suicide.
His dreams go into the overarching theme of fate, and that the fact that it's inevitable. His dream is foreshadowing what the audience already knows, the lovers fates