An extra layer of fabric that is used for either:
a)support and concealment, or in some cases, both
b) to strengthen a soft fabric and to conceal raw edges with the minimum visual impact.
In brief: the facing is cut to exactly the same dimensions as the piece to be faced, and usually from the same fabric. The two are placed right sides together then stitched together, the seams trimmed and the facing pressed to the wrong side, leaving a neat edge on the front and the raw edge enclosed and hidden.