it's a cache aerator. The name indicates that the aerator is tucked up inside the faucet spout, and if you need to remove it, which you do from time to time, you can't do it with a wrench. The cache aerator
Note: faucets may have different sizes, a converter looks like a washer with threads inside and out. Note direction: you are usually above the faucet, looking down, so clockwise/counterclockwise orientation for tighten/loosen may appear backwards. Step 1: Remove old end, aerator (looks like a...