A lunar eclipse happens when Earth passes directly between the moon and the sun. During a lunar eclipse, the face of the moon appears to turn reddish as Earth occludes the sunlight that is normally hitting its surface.
A lunar eclipse can only happen when the moon, Earth and sun are all lined up exactly, or almost exactly. This is called "syzygy," and can only happen during a full moon. The moon appears reddish during the eclipse because, as Earth passes in front of it, Earth's atmosphere filters out the green to violet portions of the sunlight, leaving only the red portion, which can be seen in the shadow that is cast.