If you want to know just about the line you quote, "But soft! What light through yonder window breaks?" Romeo is personifying the light and giving it the power to break through the window. This light is actually the candle light and you are to assume it is the light that is also illuminating Juliet as she emerges onto the balcony.
Romeo then states, "It is the east, and Juliet is the sun!" "It" refers to the balcony, therefore the balcony symbolizes an eastern land (eastern also representing the exotic) and Juliet is the sun that is rising out of the east, or literaly walks out onto the balcony (remember, the sun rises in the east and sets in the west).
Shakespeare uses light and dark images to symbolize good and evil. Juliet, being the brightest object, the sun, represents the best of all good things.