Assigned male at birth (AMAB) is used to indicate that a person was labeled male when they were born. Often, it refers to people whose bodies have male sex characteristics (such as a penis and tesicles), but it can also refer to people who are intersex and labeled male.
More About Assigned Male at Birth (AMAB)
This term was developed in response to the idea that trans individuals "really" are a gender other than the one they are or were born another gender. Male assigned at birth (along with female assigned at birth) indicates that the individual in question was assigned a gender by an outside force when they were born rather than being that gender to begin with.