A zigzag stitch is sewn close to or over the raw edge of the seam allowance.
Effective at keeping the raw edges of the seam allowance from fraying. Adds little bulk to the seam allowance.
Doesn’t look as neat and clean as some other methods.
How to Make
Trim the raw edge of the seam allowance (to make a clean edge), then zigzag over the raw edge using a medium zigzag stitch. Or you can zigzag close to the raw edge first, then trim the seam allowance close to the stitching.
You can either press the seam allowance open and zigzag each edge separately or you can leave the seam allowances together, zigzag them together, and press the seam allowance to one side.