The Moon appears luminous because it reflects the light from the Sun, making the side of the Moon that faces the Sun visible from Earth. The amount of this light changes as the bodies revolve in their orbits.
During the full-moon phase, when the Moon appears as a complete circle, the Moon is on the opposite side of the Earth from the Sun. When the Moon and the Sun are on the same side of Earth, the reflecting side of the Moon is not visible from Earth. This phase is called a new moon. Between both phases, the Moon waxes and wanes as different amounts of sunlight are reflected off the Moon towards Earth.