The ISS's orbit is slowly decaying. While it might seem a permanent fixture in the sky, the orbiting space laboratory is only about 400 km above the planet. There might not be a lot of atmosphere at that altitude. However, there is still some, and interacting with that is gradually slowing the orbital speed of the station, decreasing its orbit, and, eventually, pulling it back to Earth. That is, if we didn't do anything to stop it.