Single-fold and double-fold describe how bias tape is folded. The distinction is not the material or color; it is the way the strip is prepared for sewing.
Single-fold: edges folded toward the center
Single-fold tape has both long edges turned toward the center. It does not yet have the lengthwise fold that closes the strip in half. The raw edges are tucked inward.
This format can be used for a finish that turns to the inside of a piece. Placement depends on the pattern and how much of the edge should remain visible.
Double-fold: one more lengthwise fold
Double-fold tape starts with the edges turned toward the center, then adds a fold along the length. This lets the tape wrap around the edge of the main fabric and form a visible binding on both sides.
When sewing it, check that the stitching catches the tape on both faces. A neat line of stitching on the front is not enough if the back edge is left unsecured.
Choose for the intended finish
For a finish turned to the inside, consider a construction using single-fold tape. For binding that wraps around the edge and remains visible on both sides, consider double-fold. In either case, the width needs to suit the thickness of the piece and the intended finish.
Check the available tape formats and state whether your width measurement refers to the open strip or the folded tape. A sample of the intended finish helps avoid confusion.
For specific projects, see what bias tape is used for. For the distinction between a bias cut and a fold, read what bias tape is.