During the new Moon, the sunbeams illuminate the face "hidden" of the Moon (the one facing the Sun) : from the Earth, we dont see it any more.

During the first and last quarter, the sunbeams illuminate half of the visible face (and half of the "hidden" face) : from Earth, we only see half of the Moon.

Finally during the full Moon, the sunbeams fully illuminate the visible face : from the Earth, the Moon is completely visible.