It's a newish term for the default state of the world's population, those whose sense of gender identity matches their sex at birth.
It was created to avoid the clunky "non-transgendered" and the pejorative "normal". "Cis-" is from Latin "cis", on this side of something, as opposed to "trans-", on the opposite side, which is from Latin "trans", across..
Source: Worldwidewords
