The Moon does not have any light of its own and it shines because its surface reflects sunlight. During a total lunar eclipse, the Earth moves between the Sun and the Moon and cuts off the Moon's light supply. When this happens, the surface of the Moon takes on a reddish glow instead of going completely dark.
Since the Moon only reflects light from the Sun, when the Earth blocks the way between the Moon and the Sun, no light can reach the Moon and it is darkened. This is, again, known as the lunar eclipse.
