Geostationary satellites have a circular equatorial orbit, at 35,786 km above the Earth. Launch vehicles can only place satellites in an elliptical orbit with some degree of inclination from equator. So, geostationary satellites are placed in elliptical "geostationary transfer orbits". And, satellite's onboard propulsion system is used to:

1) Gradually raise the apogee

2) Transform elliptical orbit into circular orbit

3) Reduce inclination to zero degree with respect to equator.

Orbit's apogee is raised by firing onboard motor at perigee. Circular transformation is done by firing onboard motor at apogee.