When Mercutio goes on a rant about dreams, what is he trying to say or the deeper meaning behind it?

Are you referring to his Queen Mab speech? The speech has to do with dreams and sex. The speech is based on pagan Celtic mythology. Mercutio’s speech is laced with sexual innuendo. The words “queen” and “mab” refer to whores in Elizabethan England. As his speech goes on we notice the subtext get increasingly sexual culminating with Mab teaching Maidens how to have sex. Mercutio gets pretty hot and bothered by his own rhetoric . On the surface he means to poke fun at Romeo’s romantic obsessions. Mercutio gets all sweaty and intense by the end of his speech. Romeo attempts to calm his friend down, “Peace Mercutio…you talk of nothing.” We get the sense that there is something way more personal here than simply a prostitute fairy who teaches maidens to have sex. Mercutio clearly wants Romeo for more than hanging out by the tomato stall. There is pent up frustration on Mercutio’s part both emotionally and sexually.