RCS also stands for Reaction Control System, a system of
thrusters on board
spacecraft which allows
attitude alterations.
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.