There are many definitions for “fob.” In the case of keys, a fob is also called a keychain.
In other words, it’s metal accessory that attaches to your belt or lapel which can hold your keys. It might also be encased with things like stainless steel bottle openers and led light pens to make it more useful on various occasions. All in all, it’s an excellent way to keep your keys in order from being lost down some microscopic little crevice between the couch cushions or underneath a pile of clothes on the floor (which is if you’re forgetful like me). The best thing about a fob is that if get scratched up or dull looking, swapping out its contents for new is very easy.