A tumbler is a big bowl looking thing with a motor in the base. You put tumbling media (found wherever reloading goods are sold) in it, switch it on, and it cleans the brass by the abrassive action of the media. It makes the brass look all purty and stuff, but really is not of major importance unless you end up with real dirty brass, making it hard to check for any neck splits, or problems in the shoulder, especially when getting into the higher counts of reloading that same piece.