Picture rolling a ball up a hill. If you don't roll it fast enough it will roll back down before reaching the top. If you roll it too fast it will go over the top and roll down the other side. Just right, and it will stop at the top of the hill. But being at the top of the hill, it's an unstable orbit, and any small drift will end up with the ball eventually rolling back down the hill. That's why occasional course corrections are required, and why the lifetime of the satellite is determined by the amount of fuel it has.