A nipper is used to cut your fishing line, like your leader or tippet, and to cut excess line after tying on a fly.
Just like some forceps, a nipper can have a little spike. You can use this if the eye of the hook on your fly is obstructed, mostly caused by excess glue from fly tying, by pushing the spike through the eye of the hook.
Want to read more about fly fishing accessories? Check out our list of the essential fly fishing accessories.