"Facing" - in clothing construction -
See attached - it's a way to finish a neckline or an armhole.
A hem is actually sort of a facing.
Although why it's called "facing" puzzles me - it's usually on the back/inside of a garment and on the back of an item.
After the facing is sewn to whatever (right sides together) - then the curves are clipped and the facing is turned to the back.