An analogy is a comparison of two unlike things based on the resemblance of their particular aspect. Its purpose is usually to explain or clarify something.
Here, Juliet uses an analogy to compare a rose and Romeo. A rose and Romeo don't have anything in common - a rose is a flower, and Romeo is Juliet's lover. However, she has found a similarity between them. The smell of a rose is as sweet as Romeo, and that is why she makes this analogy.
