When one side of the moon lights up what does it mean?

Whichever half of the moon is facing the sun at any given time will be illuminated by sunlight; this is exactly the same process which gives us day and night on our own world. Because the sun, moon, and Earth are all moving in space and changing their relative positions with respect to each other, our view of the moon changes, from the full moon in which all of the illuminated portion faces us and is visible, to the new moon in which the illuminated side faces away from us and is not visible.