Spacecraft --- Space Station --- Propulsion

The ISS orbits the Earth at an altitude of 362 to 475 km. Although the Earth's atmosphere is thin at this altitude, it is still thick enough to slow the ISS down. When the ISS slow down, it loses altitude.
So the ISS will need boosting periodically to maintain its proper altitude. The command and service modules have rocket engines that can be used to boost the ISS in the early stages.
However, the Progess supply ships will do most of the reboosting and each reboosting requires two rocket engines burns.

Structure of the ISS Propulsion Communication Navigation Power Computer Resupply Escape Research in the ISS