Sun-synchronous orbit (SSO) is a particular kind of polar orbit, see picture below.
Satellites in SSO, travelling over the polar regions, are “synchronous” with the Sun. This means they are synchronised to always be in the same ‘fixed’ position relative to the Sun. This means local time is always the same then it crosses the equator (or any other latitude). Note that it does not mean it crosses over a particular location every day.
Having a constant local time of passing over the earth allows researchers to get consistent data – i.e if images were collected over several weeks with times from 1:30 pm and then 1:30 pm the land would look very different – and many features may well be hidden in the dark.
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.