A geostationary satellite orbits the planet at the same rate as the planet rotates, so the satellite is always over the same patch of ground.
Cool! you say. I'd like to have one of those about twenty feet over my house. Sadly, geos only work easily at one distance: 22,500 miles up.
Why is that? Other satellites can have different orbits, can't they?
Yes, they can. But that's because they can also move at different speeds. A geo doesn't have that option -- by definition it must orbit at the same speed as the earth turns. And that speed determines how high up it must be!
A satellite has to move at a certain speed to balance out gravity; otherwise it won't stay in orbit. Now, a geo must move at the speed the earth turns. If you put it in a very low altitude, that speed wouldn't be enough to balance gravity.
Go higher, though, and gravity gets weaker. At 22,500 miles, gravity will be just weak enough so that a satellite moving at the speed of the earth's rotation will stay in orbit without help.
Of course, if you wanted to have a geo at a different altitude, you could always attach rockets to it and have them push continuously. But that's a pretty expensive option. At 22,500 miles, nothing else is required.