Buttonhole stitch is a looped stitch used in applique work and to stabilize edges in button holes.
Bring the thread up at A. Go down at B and come back up at C. Note how the working thread is below the needle.
Repeat by going down at D and coming back up at E.
When the stitches are close to each other, this stitch is called buttonhole stitch. When the stitches have gap between them, they are called blanket stitches.
Completed work will look like this.