This scene in Romeo and Juliet is defined by Romeo's use of lofty, figurative, courtly language, which stands in contrast to Juliet's generally more down-to-earth speech. Romeo uses the language of a young lover in court, which causes him to use metaphors such as this one:
With love's light wings did I o'er-perch these walls
In answer to Juliet's serious inquiry as to how he came to be in her father's orchard, Romeo says that he has flown over the walls of the orchard on the wings of love. In this context, the wings are metaphorical: Romeo is saying that being in love with Juliet gives him the sensation of being able to fly, or walk on air.
When Juliet protests, very rationally, that this will not save Romeo if her family finds him here, Romeo says, "I have night's cloak to hide me from their sight." In this statement, it is the "cloak" which is metaphorical. Romeo is saying that Juliet need not worry, because he will be concealed from the sight of any who would be alarmed by his presence by the "cloak" of darkness.
Romeo also uses an extended (hyperbolic) metaphor to describe the moon early in the play, when he says that "her vestal livery is but sick and green." Here, he casts Juliet as the moon's maid and commands, "be not her maid, since she is envious." This metaphorical language is used to praise Juliet's beauty, saying that she is fairer than the moon itself and that the moon is "sick and green" with jealousy over this fact.