In Romeo and Juliet, William Shakespeare frequently used characters as foils, which simply means that those characters were used to contrast other characters. In Act II of the play, Mercutio acts as a foil to Romeo, while the Nurse is a foil to Juliet.
In Scene V of Act II, Nurse arrives back at the Capulet home after having met with Romeo. Juliet, who is desperate for whatever news Nurse has for her, is impatient and demanding; she cares about nothing more than her marriage and has no interest in, or patience for, other topics. Nurse, however, is worn out from her excursion and complains that her bones ache, she is out of breath, and her head and back hurt. Although Nurse does have the news Juliet wants so badly, Nurse prefers to ramble and "beat around the bush" before she delivers Romeo's message.
Mercutio acts as Romeo's foil in Scene IV. Mercutio tries to impress on Romeo the belief that it is much more fun to enjoy life that be depressed about love all of the time. Mercutio does not realize that Romeo is no longer heartbroken over Rosaline.