It is a ruffler. You can use it to gather fabric or make pleats depending on how large you set the top gage.
A small drop of oil on the hinging parts(wipe off excess) will help it move freely. The one in the photo is designed for a low shank machine- the distance from the base of the fot to the top of the area that clamps to the machine presser foot bar is how you know. Singer Co has free downloadable directions on their site. Directions for these is pretty standard- They have multiple applications and can be a great resource