This is a steel rod for cleaning the inside of piston valve casings. It's most commonly used for trumpets, but euphonium and tuba pistons need love too. A slot in end of the rod will allow you to attach some cheesecloth or other soft cloth. Simply remove your piston and the bottom cap and you can slide this cleaning rod through the casing to remove dirt and grime. With regular cleaning, you'll keep valve casings cleaner for more reliable valve action and longer life. With a thin cloth over the rod, you may be able to clean inside some first and third slide crooks, but do use care due to the small size of the tubing. Tubists with movable first or main slides may find this short rod usable for cleaning those parts.
If you're a trumpeter, there are only a couple of other cleaning aids that you might consider to keep your instrument clean. One is a swab to clean the leadpipe. Bob Reeves makes one of the best. The other item is a flexible snake which will clean most anywhere inside your trumpet.