This is a half inch hem roller that assists in the folding material into a double fold before it's fed into a sewing machine.

The channel is 0.5mm, which accommodates thin materials such as 1.1oz nylon.

Here is a video of it in action:https://www.youtube.com/watch?v=YL3GEMx8yjs&feature=youtu.be

There is a small tongue on one end which is used to initially insert the material into the roller. Initial insertion is quite tricky- you need to use the tongue as something to 'press' against as you pinch and roll the material into the final fold.

Make sure the tongue is not facing the sewing machine or the foot could break it.