A pipe cleaner is, not surprisingly, a device used for cleaning pipes.
But don't let this confuse you. Don't go searching for pipe cleaners at your local DIY warehouse. Because they're not for cleaning water pipes or sewage pipes, they're for cleaning tobacco pipes. Go to a tobacconist!
(OK, I'm sure there are devices for cleaning sewage pipes that are also called pipe cleaners but that's not what I'm talking about here!)
A pipe cleaner is about 6 inches long and has a core of twisted metal (which makes it flexible enough to bend, but firm enough to retain its shape), coated with what's best described as fluff. This allows it to be passed up and down inside a pipe to clean up tobacco, spit or whatever else may end up inside your pipe.
Pipe cleaners are cheap - the last pack I bought was about £0.50 (under US $1) for a pack of about 50. And no, I don't smoke, but for the Jewish festival of Chanukah, I light a Chanukiah which uses olive oil. And pipe cleaners cut into 1.5 inch lengths make wonderful wicks!