Answer:
In this passage, Shakespeare makes a pun, or a play on words, with the word "lie." In this passage, Romeo is warning his friends that he thinks it is unwise to crash the party because he believes he has had a prophetic dream predicting his own death as a result of attending the party, as we see later in his lines,
With this night's revels and expire the term
Of a despised life, clos'd in my breast,
By some vile forfeit of untimely death. (116-118)
The "despised life" that is "clos'd in my breast" refers to his own life, because only his own life, his heart, lives in his own breast. Thus, we see that Romeo is beginning to tell Mercutio and Benvolio about his prophetic dream that he has interpreted as a warning, when he says, "'[T]is no wit to go...I dreamt a dream to-night" (51-53). Mercutio wittily replies that he had a dream as well, and his was that "dreamers often lie" (56). The word "lie" has a double meaning here. It can refer to being in a "horizontal, prostate position," as in lying down (Random House Dictionary). However, it can also refer to the act of not telling the truth, deceiving, or giving a "false impression" (Random House Dictionary). Mercutio is using it to say that dreams are deceptive; they do not speak the truth. However, Romeo, creating the play on words, interprets "lie" to refer to lying down; therefore, he says, "In bed asleep, while they do dream things true" (57). In other words, Romeo is saying that dreamers lie (as in lie down) in bed, while they are dreaming things that are actually true.
Hence, we see that the play on words, or pun, is created from Mercutio's use of the word "lie," which Romeo interprets to mean both lying down and telling the truth.
