A cisgender person is a person whose gender agrees with the sex they were assigned at birth. The term was created for referring to "non-transgender" people without alienating transgender people. Some people exclude intersex people preferring the term Ipso gender instead.
Cisgender may sometimes be referred to as cissexual (corresponding to transsexual, not to be confused with a sexual orientation), or shortened to cis.
Cis as a prefix directly means "on the same side of," thus cisgender being identifying your sex to be the same as your birth. You may still use different pronouns, but pronouns aren't a direct meaning of gender.