AMAB: an acronym that means “assigned male at birth”.
It means the sex of a baby by examining the genitalia, either in an ultrasound or once the child is born.
AMAB itself is not a gender. You can be AMAB 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 AMAB further distinguish biological, assigned sex from lived and expressed gender.