What does Dark Souls 3's Season Pass do?

The Season Pass is the two DLC. The wording is not perfectly clear, but by buying the Season Pass you don't gain access to the ability to buy DLC at a discounted price, you gain access to the DLC itself, at a lower price than buying them individually.

Incidentally, if you wanted to buy both Dark Souls III and the Season Pass, this package would be the cheapest option on Steam right now.

Season Passes usually offer all current and future DLC in one, often discounted, package. Unlike GOTY deals, for example, they do not include the base game.

The Dark Souls III Season Pass gives you both the Ashes of Ariandel DLC and The Ringed City DLC.