This is a very simple tool that helps with tying a loop knot. See more information on how this is used here:

Tool is very small and has sharper edges than those you can purchase. I did this on purpose to simplify the model and printing configuration. You can bevel it if you wish but in my experience it didn't cause any damage to the fishing line if you don't tighten too much. Also it's important to mention that this tool makes rather small loops. Scaling the tool scales the final loop size.

Note: Video is not mine but author did a good job of explaining how this tool is used and video is very short and to the point.