The Latin prefix cis- , meaning “on this side of”, which is the opposite of the prefix trans- . So a cisgender person is someone whose gender identity matches the sex assigned to them at birth - I.e. someone who is not transgender.

They’re value neutral words and are useful words to have. For example, if I wanted to make the point that not everyone who cares about trans rights or gender issues are transgender, I could point to myself, as a cisgender woman.