Juliet feels that names ultimately have no real meaning. A thing is what it is, and nothing about a thing is bound up in its name; a thing still is what it is whether it is called by a particular name or not.
When she learns that the man with whom she's fallen in love at first sight is the son of her family's sworn enemy, she reflects on the fact that Romeo is many things, but he is not his name. His name is not a part of himself, and only his name is her enemy. After all, she ponders aloud, a rose would always smell sweet whether it is called a rose or not. Similarly, Romeo, to her, is perfect, and his perfection has nothing whatsoever to do with his name. She attempts to justify her feelings and attraction to this man that she knows she is supposed to hate by reasoning that his name -- which she is supposed to hate -- is not the same thing as his person (which she does not hate).