“CIS” people (whatever that means, I genuinely don’t know, I don’t keep up with ideological abbreviations)

Cis is a prefix. It is the opposite of trans. In this context it’s obviously an abbreviation for cisgender va transgender.

Cis means “this side,” trans means “the reverse side.” You see them used in chemistry, for example cis/trans-isomerism. Even Julius Caesar’s writings reference cisalpine Gaul and transalpine Gaul (the part of Gaul on this side of the Alps vs the part on the other side of the Alps)

Hope that helps you “keep up” with the English language.