What is a season pass?
Season Passes usually offer all current and future Downloadable Content (DLC) in one, often discounted, package.
Unlike GOTY deals, for example, they do not include the base game.
Season Passes are usually tied to a consumer's account of a digital distribution platform, like Steam, Origin, GOG Galaxy, or Uplay, so that upon release new content can automatically be downloaded to their accounts.
In rare cases, Season Passes offer exclusive additional content.
- One the one hand, Season Passes guarantee the consumer that their game(run) has a longer lifespan, since they will automatically get future DLC.
- On the other hand - and this is why Season Passes have at times caused controversy - the DLC upon release might turn out to be of much lower quality than the base game, leaving consumers disappointed with their purchase.
Some Season Passes only include DLC from a certain time window. In that case they are only valid for a limited time, and all DLC released after this window has to be bought separately.
Additionally, even if all DLC is worth it, once most of it has been released, new deals might exist that easily outweigh the Season Pass in terms of price and value. Newer deals can even include more DLC for a lower price.
Whether or not they are worth buying, can only really be determined after most or all content for a game has been released or at least announced, as this gives a clearer idea of the quantity and quality of the extra content the Season Pass offers.
Basically, it is a way of saying "Oh hey, here is not only our awesome game, but also all the awesome DLC we are going to release (or have released, or both)".
It's not something exclusive to Steam - I have seen it used in other places as well.