There is such a thing as a person’s sex, right? It might be on a continuum, rather than a binary, but you’d agree that there is a biological set of sex traits, right?
I assume you’d also agree that all people identify as having a gender. There can be many different genders. Even agender is an aspect of gender identification defined by the lack of gender identification.
OK, if you are with me this far, when someone is content that their sex (however defined) is aligned with the gender identity they have, then they are cisgender. If there is dysphoria they are transgender.
It is a clinical distinction and being way overused for social and political purposes.