Gosh, I just LOVE this scene in Romeo and Juliet! The original answerer is spot-on; however, I would like to add that we share in the nurse's joke in this scene thanks to dramatic irony. We know something that Juliet doesn't yet: that Romeo prepares to be her husband!
This is the scene where the audience begins to truly understand the very tender, dear, and loving relationship that Juliet has with her nurse. The nurse understands just how much Juliet is looking forward to this news. Further, the nurse already knows the news is good (in other words, that Romeo truly does want to marry Juliet); therefore, she shows her awesome sense of humor by making Juliet wait for it. Even further, the nurse gets joy out of hearing Juliet's reactions:
Juliet. What says he of our marriage? What of that?
Nurse. Lord, how my head aches! What a head have I! / It beats as it would fall in twenty pieces. / My back o' t' other side--ah, my back, my back! / Beshrew your heart for sending me about / To catch my death with jauncing up and down!
Juliet. I' faith, I am sorry that thou art not well. /Sweet, sweet, sweet nurse, tell me, what says my love?
Nurse. Your love says, like an honest gentleman, and a courteous, and a kind, and a handsome, and, I warrant, a virtuous--Where is your mother?
Juliet. Where is my mother? Why, she is within. / Where should she be? How oddly thou repliest! / "Your love says, like an honest gentleman, 'Where is your mother?'"
The nurse's humor goes on and on, making all of the audience smile. It's good, I suppose, that we have some comic relief here, for the play is so consumed with suspense and nail-biting for most audience members. In my opinion, the version with Claire Danes and Leonardo Dicaprio is phenomenal during this particuar scene. Both the tenderness between Juliet and her nurse as well as the substantial humor of the scene are unparalleled.