The moon appears bright because the side visible on Earth reflects the light from the sun. As the moon and the Earth move around their respective orbits, different portions of the moon are visible to people on Earth, depending on the angle at which the light hits the moon and reflects back to Earth. The moon reflects back about 12 percent of the light from the sun that hits it.
The moon appears brightest during the full moon phase of its cycle. During a full moon, the moon is directly opposite the sun, and a full hemisphere reflects back the light. During the opposite phase, known as a new moon, the moon is located on the same side of Earth, and the sun and none of the illuminated portion is visible on Earth. Next to the new moon phase, the moon is at its least bright during the waxing crescent and waning crescent phases, when only a sliver of the illuminated portion is visible on Earth. The next brightest states are the first and last quarter moons, when about half of the illuminated hemisphere is visible. In between a full moon and the quarter phases, the waxing gibbous and waning gibbous phases display less than the whole, but more than half, of the hemisphere illuminated by the sun.