If you're making a shirt or  jacket, chances are you're going to have to put a facing in it.  A facing is a thing strip of fabric with interfacing that can be sewn along the front or around the neck or around armholes. In the case of this lightweight jacket it goes around both the neckline and front  A facing is a way to finish those edges but also strengthen and stabilize the area.  It also supports areas that will have buttons and button holes, snaps, and zippers. Lots of garments have a facing and if you add them, it'll make your stuff look even more professional. Watch to see how it's done.