The phenomenon known as Earthshine is reflected Earthlight visible on the Moon's night side. It is also known as the Moon's "ashen glow" or "the old Moon in the new Moon's arms."
Earthshine is most readily observable shortly before and after a new Moon. When the Moon is new as viewed from Earth, the Earth is fully lit up as viewed from the Moon. Sunlight is reflected from the Earth to the night side of the Moon. The night side appears to glow faintly and the entire orb of the Moon is dimly visible.
Earthshine is used to help determine the current albedo of the Earth. The data are used to analyze global weather phenomena. Oceans reflect the least amount of light, roughly 10%. Land reflects anywhere from 10-25% of the Sun's light, and clouds reflect around 50%. So, the half of the Earth where it is daytime determines how bright the Moon's Earthshine appears at any given time.
See also: André-Louis Danjon, Danjon scale.