Its like the open toe darning. some are plastic and some are metal, I prefer clear plastic if I have a choice. The bottom of the foot is a circle with the front part cut out so that you can see what you are doing better. It can also be square like the open toe satin foot. Different machine companies make the differently. the youtube video will show a picture of it. All of the ones that I googled won't let you copy their pics. Google it and I'm sure you'll see lots of pictures of it.
https://www.youtube.com/watch?v=EEKE1D_kmCo