AFAB: an acronym that means “assigned female at birth”.
It means the sex of a baby by examining the genitalia, either in an ultrasound or once the child is born.
AFAB itself is not a gender. You can be AFAB and identify as trans, non-binary, agender, or any other gender.
No one, whether cis or trans, gets to choose what sex they’re assigned at birth. Terms like AFAB further distinguish biological, assigned sex from lived and expressed gender.