The Moon does not emit its own light, shining instead by reflecting sunlight. Depending on the relative positions of the Earth, Sun and Moon, varying amounts of the lunar surface appear illuminated. A Gibbous Moon occurs a few days either side of a Full Moon. The illuminated part of the moon is greater than a semicircle and less than a circle and can be either a:

Waxing Gibbous Moon: the phase before the Full Moon, where the amount of the Moon’s disk that is illuminated grows in size over time (position D in the image below).

Waning Gibbous Moon: the phase after the Full Moon, where the amount of the Moon’s disk that is illuminated reduces in size over time (position F in the image below).

The Moon’s motion around the Earth, with the Sun illuminating only one side of the Earth and Moon.