The Nurse has looked after Juliet from her babyhood, and she is extremely supportive of Juliet throughout, to the extent that Juliet seems to trust her more than she does her own mother. Throughout the early part of the play, the Nurse is supportive of Juliet's love for Romeo, which she has been told about in confidence; she even knows that Juliet and Romeo have been married in secret. This makes the advice she gives to Juliet in Act 3, scene 5 particularly difficult for Juliet to accept.
Juliet asks for some "comfort" from her Nurse because of the fact that Romeo, now her husband, has been banished. Instead of offering any real comfort, the Nurse tells her that she should be "married with the county." She is suggesting that Juliet commit bigamy, for purely practical reasons which do not take into account Juliet's feelings. The Nurse says that because Romeo is banished and is therefore "nothing" to Juliet, it is the best thing she could do to marry Paris, by comparison with whom Romeo is a mere "dishclout." She is trying to comfort Juliet as best she knows how: by saying that Romeo isn't particularly great anyway and that Paris is "lovely" and will be a good match for her. However, Juliet clearly feels rather betrayed by this advice.