A lunar eclipse happens when the moon enters the Earth's shadow, i.e. the Earth is between the moon and the sun. This casts the moon into darkness because it no longer has the sun to light it up. Because the moon is in orbit around the Earth, the moon's entering the Earth's shadow is a temporary thing, and every so often will move out of it.

The moon (as well as the sun, and the rest of the stars in the sky) appears to change its position in the sky because of the way the earth rotates around an invisible axis that is slightly tilted from from the line that would go from the north pole to the south pole. To help you see why this is true, take a balloon and draw a dot on one side of it. Image the dot is where you are standing on the earth. Then, shine a flashlight on the balloon. Spin the balloon and you will see that the side of the balloon with the dot will alternately be lit up and then be dark. From the point of view of the dot, it looks like the light is "rising" and "setting" -- just like the moon, sun, and stars appear to rise and set for us!