The prefix cis- is Latin and means "on this side of".
It's been used by activists since 1994 to serve as the opposite of trans, means "across or beyond".
I'd say it's made up for political purposes but others might say it's Latin, so "it's real". I don't accept it. And I don't say "gay" either. I say "homosexual" to describe the unhappy people suffering from a mental illness and perversion.
The only place I ever saw the word cis- before the lunatics hijacked it was in the name of the Roman province of Cisalpine Gaul.
Thanks to your post I now know what the name means!