Romeo speaks these words soon after he has jumped over the wall of the Capulet's garden. He wishes to see Juliet again for he is completely infatuated with her. Romeo is standing in the garden when Juliet appears at a window above.
He compares Juliet to a light, using a metaphor to describe her. The fact that he sees Juliet as a light implies that she has enlightened him (brought a new perspective), and brought light back into his life. We must remember that he had been quite depressed about Rosaline having rejected his advances and Juliet has brought back his joy: she has lifted the heavy and sombre mood that he had felt. She has driven away the darkness caused by the despair that he has been feeling because of Rosaline's cold and indifferent attitude.
The metaphor is extended when Romeo mentions that the position from which the light emanates is the east, therefore indicating sunrise. His meeting with Juliet has introduced a new dawn into his life, a new chapter. His life has been refreshed and the dawn signifies a new birth, a new start, which is exactly what Romeo experiences at this point.
Furthermore, the metaphor signifies that Juliet is "the sun": she is the bringer of life, the energy that has revitalized him. Juliet symbolizes a renewal in Romeo's life. She has brought him a new energy. His love for her has re-energized him. There is a new purpose to his existence and Juliet is the source of all this vitality.
This is an example of a metaphor, and even personification. Here's how:
But soft, what light through yonder window breaks - this is a stretch, but could be explained through personification in that light is not a human and is an inanimate object and therefore cannot break through anything.
It is the east, and Juliet is the sun - Here, Shakespeare makes a metaphorical comparison between Juliet and the sun. (It is not a simile as it doesn't use like or as.) Furthermore, a metaphor is a comparison between two entities that are not alike in any way, except in the aspect that they are compared. How are Juliet and the sun alike? They are both bright, and they are both something light in an otherwise dark night.
This scene, as does the rest of play, uses a lot of light/dark imagery to convey just how outstanding and extraordinary Romeo believes Juliet to be.