On a typical day, drag from the thin atmosphere causes the station to drop about 300 feet in its orbit. If nothing were done, eventually it would dip into even thicker air and start to break apart.
So every three months or so, the station needs a boost. Typically this is done after a cargo ship like the Russian Progress or Europe's Automated Transfer Vehicle (ATV) docks with the ISS. The visiting vehicle, plugged into the Russian Zvezda Service Module, at the station's aft end, burns its engines for one to 20 minutes, increasing the station's orbital speed and therefore its altitude. An acceleration of a few feet per second is all it takes to raise the orbit by a few miles.
Zvezda also has thrusters—and can itself be used for reboost or large maneuvers beyond the capacity of the CMGs. On rare occasions, when some piece of space debris is predicted to come uncomfortably close to the station, the orbit can be raised slightly to reduce the chance of collision.