The term "amab" refers to Assigned Male at Birth, which is a term used to describe individuals who were identified as male at birth but may identify differently now. In this context, it's referring to an individual with a male assigned gender at birth who identifies as polyamorous.
Polyamory means the practice of engaging in multiple intimate relationships, with the full knowledge and consent of all people involved. So, "amab-polyamorous" is describing someone who was assigned male at birth, who also identifies as polyamorous.