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 is sewn along the front and goes around the neck.  You will also commonly find facing on necklines that are collarless or armholes that are sleeveless,  A facing is a way to finish those edges but also strengthen that area.  It also supports areas that will have buttons and button holes, but also helps finishes your garment so that it comes out looking professional.