The moon does not create its own light, but the sun reflects light of the moon so depending on where the sun sits (which it is always moving) causes the different phases or appearances to the moons shape
The half of the moon that points toward the Sun looks bright because it is lit by sunlight. The moon appears to change shape because we see different amounts of the lit part as the moon orbits Earth. When the moon is between Earth and the Sun, the lit side is hidden from you.
