Bias binding tape is a strip of fabric, cut on the bias (diagonal) of the fabric, that is used to enclose and finish the edges of a garment or other sewing project. It is typically made of cotton or a cotton blend (but can also be made of a silk or polyester satin) and comes in a wide range of colors and patterns.

The bias cut of the tape allows it to stretch and curve around the edges of a project, making it ideal for finishing curved or irregularly shaped edges. It is also useful for creating a neat and professional finish on the inside of a garment, as the tape covers and secures raw edges, preventing fraying and ensuring a clean edge. 

Bias binding is commonly used to finish the edges of garments, such as necklines, armholes, and hems, as well as on quilts, bags, and other sewing projects.