A planetarium is an enclosed room used to simulate the celestial sky. This is done by means of a star projector and other smaller projectors for planets, the moon, the sun, and other astronomical features. This is different from an Observatory where one would look through a telescope to see a small fraction of the sky. In the planetarium, it is like you are standing outdoors in a large open field.
We use the planetarium as a teaching tool to show how the sky changes during the course of a day or even over a longer period of time. With the proper information, we are able to set the planetarium to dates in the past or into the future.