It is a Walking Foot; used when you quilt or when you have a thick layer of fabric to sew on. I usemine with my Viking SE I sewing/embroidery machine.

This is a walking foot. You use it in quilting or when you have a heavier weight of fabric, for instance, the cotton fabric and two layers of batting. You can use it daily instead of the normal presser foot. You can set the needle on the right side, the middle, or the left side. I have a Viking Designer I embroidery/sewing machine.