Sometimes you will pull up bits of junk when you are fishing. Your first inclination will be to throw it out in the recycle bin. This is a good option as having junk around will make your town rating go down.
There is another use for it in the game, however.
Drop the junk on the ground and let it fester there for a while. Soon you will have flies buzzing around the trash. Get out your net and grab yourself a fly. It isn't worth much to Tom Nook, but it will help you round out your collection at the museum or of course you can keep it as a pet in a jar in your house.