RCS also stands for Reaction Control System, a system of thrusters
on board spacecraft
which allows attitude
Usually using gaseous helium, RCS jets allow spacecraft to be rotated in space, usually as part of thermal stabilization manoevres (keeping the spacecraft at a uniform temperature
), or to align the spacecraft main engines prior to a burn
RCS jets in themselves are (in most cases) not powerful enough to execute course corrections, although they may be used to null out residual velocity components after a manoevre.