Space, especially in near-Earth orbit, is not really a vacuum. The atmosphere is thin, sure, but it's there. Any object orbiting the Earth only a few hundred klicks up is plowing through that ethereally thin stuff, which causes drag on it. This robs the object of energy, and the orbit lowers. That in turn drops it into thicker air, which increases the drag, which speeds up the process. Unless action is taken, a fiery fate awaits.
The International Space Station is in such an orbit. Every day it loses altitude due to drag, and left on its own would burn up in the Earth's atmosphere in a matter of years. The European Space Agency, however, has built the Jules Verne Automated Transfer Vehicle, which is capable of boosting the orbit of ISS to safer heights. And that's just what they did a few days ago, lifting the station's orbit by 4.5 kilometers.