Facings are fastened to insulation materials during the manufacturing process. A facing protects an insulation's surface, holds the insulation together, and facilitates fastening. Some types of facing can also act as an air barrier, radiant barrier, and/or vapor barrier. Some even provide flame resistance.
Common facing materials include kraft paper, white vinyl sheeting, and aluminum foil. All of these materials act as an air barrier and vapor barrier. Aluminum foil can also act as a radiant barrier. Your climate and where and how you're installing the insulation in your home will determine what type of facing and/or barrier, if any, you'll need.
Some of the same materials used as insulation facings can be installed separately, bordering the insulation to provide an air barrier, vapor barrier, and/or radiant barrier.