The phase of the moon is the appearance of the illuminated (lighted) portion of the Moon as seen by an observer, usually on Earth.
The Moon exhibits different phases as the relative position of the Sun, Earth and Moon changes, appearing as a full moon when the Sun and Moon are on opposite sides of the Earth and as a new moon (dark moon) when they are on the same side. The phases of full moon and new moon are examples of syzygies, which occur when the Earth, Moon, and Sun lie (approximately) in a straight line. The time between two full moons (a Lunar month) is about 29.53 days (29 days, 12 hours, 44 minutes) on average.
The Phases of the Moon
Simulator courtesy of University of Nebraska-Lincoln