This quote comes from Shakespeare's 'Romeo and Juliet,' and Romeo says this line in reference to Mercutio, meaning that Mercutio has not been in love and does not know what Romeo is truly feeling. Romeo says this line in Act 2, Scene 2 when Mercutio is teasing Romeo about his love scars.
After Romeo delivers this line, the famous balcony scene follows. In balcony scene, Romeo is beneath Juliet's window and Juliet gives her soliloquy in which she expresses her feeling for Romeo without being aware that he has overheard. Romeo steps out from the darkness and towards the end of this scene, they make plans to marry after which Romeo leaves to find Friar Laurence.