Wikipedia
Bias tape or bias binding is a narrow strip of fabric, cut on the bias. The strip's fibers, being at 45 degrees to the length of the strip, makes it stretchier as well as more fluid and more drapeable compared to a strip that is cut on the grain. Many strips can be pieced together into a long "tape." The tape's width varies from about 1/2" to about 3" depending on applications. Bias tape is used in making piping, binding seams, finishing raw edges, etc. It is often used on the edges of quilts, placemats, and bibs, around armhole and neckline edges instead of a facing, and as a simple strap or tie for casual bags or clothing.
Commercially available bias tape is available as a simple bias tape, single-fold bias tape, and double-fold bias tape.
Single-fold bias tape is bias tape with each raw edge folded in toward the center, wrong sides together, and pressed. Double-fold bias tape is single-fold bias tape which has been folded in half and pressed, with the single folds to the inside. (Another way to think of it is to fold a Single-fold bias tape in half along its center-line.) Devices are available commercially to aid the home sewer in making folded bias tape. The fabric strip is fed through the device, which folds the fabric. The folds are then pressed into place. The resulting folded tape will be 1/4 the width of the original fabric strip.