It has already been stated in the comments that this is a bias binder attachment for a sewing machine.
I thought I would add that it will also work on a serger/coverstitch machine. I have a generic set of these in several sizes and use them to bind with bias and also to make straps either on the sewing machine with twin needles or on the coverstitch with two needles. It will work with wovens and stretch fabrics. Stretch, such as lycra and stretch velvet take a little practice to get consistant binding or straps. Also note that stretch fabrics won't need to be cut on the bias as woven fabrics will.
The upper piece on the binder that has the rectangular opening is where the screws will go to attach it to the machine. One needs to adjust the binder to sit right in front of the presser foot so that the bias strip is fed directly into the presser foot.

Thank you to Louise and Threads for yet another great construction tutorial!
I love both of the Threads DVD's with Louise sharing her expertise with clear instruction and Judith asking questions that many of the viewers would have. I refer back to these time and again. I certainly hope that another is forthcoming.