Multiplies the weapon damage by that amount. 100x5 = 500 damage. Always works, unlike Critical Hit damage multiplier (only works when you score Critical Hits).
And means that shoot 5 bullets at one pull of the trigger, like a shotgun do. But there's the "Doubles" SMG's that shoot 2 bullets at the same time, the "Double Anarchy" SMG's that shoots 4 bullets at the same time, and "Masher" accesory from Revolvers, that shoots 7 bullets at the same time, and of course shotguns, that vary a lot, but always have the xX damage. And, for example, the 5 shoots do 100 damage each bullet.