A simile is an analogy. In Romeo and Juliet, like many of Shakespeare's other works, the similes are lyrical, and there are lots of them in the text. Shakespeare's usage adds a lot of depth to his characters, and the use of simile is a very economic method of expressing Romeo and Juliet's love and feelings throughout the play, as well as moving the play's idioms along with the action. (It should be remembered that this is a script, as well as expression.)