Half of the moon's surface is always lit.
But the amount of that lighted surface that we see from the Earth changes as the moon orbits the Earth.

The moon revolves around the Earth every 27.3 days (the sidereal month.)
The period of time from one full moon to the next is 29.5 days (the synodic month.)
This time difference occurs because the Earth is also orbiting the sun (as the moon orbits the Earth.)