The mod is to pinball what tuning is to cars: the border between good and bad taste is you !
A mod is an addition to the pinball machine: it is supposed to embellish the board, but normally it does not change the game mechanics. The mod is created by the community, or third party professionals, but not by the pinball manufacturer. Indeed, an official accessory like a topper or a ball launcher is designed by the manufacturer at the same time as the machine most of the time.
The Pinhead community is full of talented modders who manage to enrich the world under the ice.