“cis” and “trans” are prefixes denoting on what “side” something is. “cis” means “on this/our side”, while “trans” refers to “the other side”, for example:
“Cisalpina” is how the Romans referred to their side of the Alps (modern day Italy), while “Transalpina” referred to land on the other side of the alps.
There exist certain pairs of molecules with either a “cis” or “trans” prefix, depending on whether certain identical groups are on the same side or on opposite sides, respectively.
The modern use of “cis” and “trans” is generally about gender. A cisgender person is someone whose gender identity aligns with their sex assigned at birth, while a transgender person is someone for whom that doesn’t hold true.
In this meme, the person on the right is wearing a transgender flag for a shirt, and presumably offending the cisgender person on the left by calling them cis. The meme is making fun of the fact that some cisgender people consider “cis” an insult, when it really only is a neutral and non-offensive description.