Sun-synchronous orbit (SSO) is a particular kind of polar orbit, see picture below.

SSO - Polar and Sun Synchronous Orbit
Polar and Sun Synchronous Orbit (SSO) – image from ESA.

Since satellites typically collect reflected light from the earth surface, that originate from the sun, it is important that the conditions are similar during each orbit – we are looking for changes and comparing data from different orbits. The Syn Synchronous Orbit archives this by some clever gravitational techniques. This orbit has a high inclination (the plane is angled in relation to the equator) and allows to earth to rotate beneath the satellite while maintaining a constant solar illumination angle.