It's a set of DLC you pay for before the DLC contents are known in full. You get a promise, like "4 map packs" for instance, but unless you wait until the content has been released before buying the season pass, it's not clear what those maps are. For Tekken 7, they've promised costume packs, new stages, two new guest characters, and a new game mode, plus some metallic colors for preexisting costumes. We don't know what the costumes will be, what the stages will be, who the guests will be, or what the game mode will be, but the season pass guarantees you access to those things as soon as they release, and there's usually a slight discount from what you would pay buying each item separately.
