This line should read as follows:
O, speak again, bright angel! for you are
As glorious to this night, that is over my head,
As is a wingéd messenger of heaven
To the white, upturned, wondering eyes
Of mortals that fall back to gaze on him
When he crosses the slow moving clouds
And sails upon the heart of the wind.
Romeo has been watching Juliet from below the balcony when he hears her sigh. In this aside, he refers to her as a bright angel against a dark sky; he says that she is as wonderous to his eyes (gazing up at her against the darkness) as an angel sailing upon the heart of the wind would be to mortals. Like any other man looking upon a heavenly being (such as Juliet), Romeo feels awed, and would "fall back" to gaze upon her. This is consistent with Romeo's character, for he is frequently awed by the women he is infatuated with, and tends to look upon them not as real humans but as heavenly beings (Rosaline is like the moon, Juliet, an angel and the sun). Against the darkness that covers him (is over his head), Juliet appears like a bright light. She is that, too. Prior to meeting her, darkness was not just over his head, it was in his heart. Now, he is separated from the dark sky by his vision of Juliet.
The quote,"O, speak again, bright angel! for thou art as glorious to this night, being o'er my head, as is a winged messenger of heaven..." was spoken by Romeo in Act II, Scene 2, the famous balcony scene. Juliet is saying that Romeo's name is her enemy and not his being. Romeo is hanging onto every word Juliet speaks. He is comparing Juliet to Cupid the messenger and saying they are both welcome entities to him. He feels both are sent from Heaven.
These words are spoken by Romeo after he's met and fallen in love with Juliet at the Capulet party. After the party, Romeo ditches his friends and climbs over the wall of the Capulet orchard. Juliet comes out on the balcony and is speaking her thoughts and feelings about meeting Romeo that night (soliloquy). As Juliet pauses, Romeo wants her to speak again, comparing her to an angel in heaven.