It's being used as a verb here, not a noun. I'll let you consult your dictionary on that one.

The buttons you ask about, they're really nice on my Campy leather straps, cuz they're short, so yea, keeps the strap from slipping all the way through and makes 'em easy to grab onto.

I've some Soma leather straps that are long as hell so the buttons were annoying as hell on those, cuz the button would make the strap flop around a lot and clack on my toe clips once per revolution. Had to take 'em offa those immediately. I guess what I really need to do is cut those Somas down a bit.