Why is geostationary important?

A geostationary satellite is an earth-orbiting satellite, placed at an altitude of approximately 35,800 kilometers (22,300 miles) directly over the equator, that revolves in the same direction the earth rotates (west to east). At this altitude, one orbit takes 24 hours, the same length of time as the earth requires to rotate once on its axis.

The term geostationary comes from the fact that such a satellite appears nearly stationary in the sky as seen by a ground-based observer.

In other Words a satellite that orbits a specific part of the earth while the earth is rotating so it looks like the satellite doesn't move. For example if you put a satellite over over the geographic US it will stay over the US and turn with the earth around the axis without ever loosing site of the US.