It tends to be used as a way to describe what is considered the most privileged group of people by certain rights activists groups (and also a few bigots, as with every group, there are bad eggs.) White people are considered more privileged than say, black people, because of studies showing that more black people tend to be brought up in poverty or that Mexican people tend to be brought up in worse neighbourhoods. (These are generalizations which I don't support by the way, I'm describing an external viewpoint, not my own.)
Uh, no, it's not used to describe what is considered the most privileged group, nor does it have anything to do with being white; if it did, the common expression "white cis male" would be redundant.
Cis is the antonym of trans, so cisgender is a term to refer to non-transgender people.
