A total lunar eclipse.
When the moon enters the dark shadow called the umbra it becomes nearly invisible. The Earth has a large enough umbra to cover all of the Moon if of the Moon goes into the right place. When the moon enters this shadow we call it a lunar eclipse.
There are actually three kinds of lunar eclipse.
A total lunar eclipse is where all of the Moon enters the dark umbra and itself goes dark. Usually we still see the Moon as a faint, reddish image because the Earth's atmosphere bends a tiny amount of reddish light into the umbra. The umbra would be truly dark if there were no atmosphere on Earth.
A partial lunar eclipse is when just part of the Moon reaches the umbra; the Moon then appears partly lighted and partly dark.
Finally there is a penumbral eclipse where the Moon misses the umbra but enters the partially shadowed region called the penumbra, the gray region behind the Earth in the picture. We don't see any of the Moon go dark, but it does become dimmer allowing astronomers to study more of the sky immediately surrounding the Moon.