These little things are line locks. they are used on a fishing line with a swivel. the purpose is when you hook a fish, the lock will slide down to the swivel and prevent the line from twisting. I have a buddy that requested I designed this because he was getting 3 for $10. I think that's ridiculous, so this is my own design. he has been testing them for over a year now with different materials in the ocean so far not have broken or degraded even with pla.