Welcome to the Quilting Board!
That is a ruffler. A marvelous attachment, it can make the prettiest ruffles so much easier to do.
I think its a gathering foot which can be adjusted to pleat/gather closer together or further apart.
It is indeed a ruffler . The little part that has the numbers at the top is how many tucks you want it to make. The little screw thing is how deep you want each tuck to be. It a great little gismo.
Looks like a ruffler to me.
Had one for a Viking that I had years ago.