What is a season pass in computer games?

It's just a preorder for DLC, basically, or if they're all released it's just a DLC bundle pack.

I have been playing video games for like 25 years already, and I have no clue. Whenever I see it, I am less inclined to purchase the game though.
- Me too. When I read "season pass" my mind goes to scams and games riddled with in-app purchases. No thanks.

A few answers here are mixing up season passes with battle passes.
A season pass is just a bundle of multiple pieces of DLC, often sold in advance with a schedule for when each item will release. Sometimes the contents can also be purchased individually, but buying them bundled in the pass will get you a discount. Generally this is a fair enough business model, even if the idea of preordering DLC doesn't sit right with you, you can always wait and just buy the pass once everything is released.
A battle pass is a model found in a number of live service games. After purchasing the pass, you can grind for in-game rewards. Oftentimes there's a deadline to finish grinding by, and once a pass is over you can no longer obtain the rewards and will have to buy the next pass for the next set of rewards. These are engagement bait to keep players grinding so that playercount remains high, and they rely heavily on FOMO, which makes them controversial. But they make a lot of money, so you'll keep seeing them.

A tactic by big money hungry studios to get you paying every few months or weeks to get meaningless content in their games, plus access to new meaningless tasks to complete (often referred to as "dailies") to give you a few more meaningless items in game, on top of attempting to addict you with said tasks to come back for more.
Wish I could paint it more positively, but the gaming industry is heading towards enshitification and this is a big part of it, extracting more money without providing much more content.
- How can the most upvoted reply be factually wrong. You are describing a battle pass. Season pass is a dlc bundle.
  
  Depends on the game. I've seen it labelled both ways.

It's effectively pre-order for DLC.
IE you pay us now... when we release the DLC you get it... If the DLC turns out to suck... well that's on you, if we release a different DLC then the one we announced, and delay or downgrade all the resources from the DLC we announced, sucks to be you etc....
Bottom line, at best it's kind of a kick starter done by companies with more than enough money to not need it.

Two types that i know of:
in-game live service bonus items rewarded for play time or achieving some kind of objectives.
potential DLCs for a game that may or may not be made
Both can be scummy. The former can basically be a mobile pay to win scam that drains bank accounts, the latter can be a "we decided not to add any more content, so thanks for the money and goodbye sucker!" scam that overprices a game and underdelivers.

A bunch of games these days will release content on a regular schedule. Instead of buying it all separately, you can get a ‘season’ pass to get all of it. Alternatively, many games will offer rewards for leveling up, but they’ll have two tiers of rewards, unpaid and paid. If you buy the season pass, you’ll get both tiers of rewards. It just depends on the game.
- Alternatively, many games will offer rewards for leveling up, but they’ll have two tiers of rewards, unpaid and paid. If you buy the season pass, you’ll get both tiers of rewards.
  That's a battle pass.
  
  I mean, a lot of games still call them season passes. Destiny 2, for instance.

They are things to avoid. Things that you don't need. Gimmicks. A waste of money.

Thanks for clarification. I was observing season passes occasionally during the years and that was my impression but I just could not believe it to be true, I thought there’s something more to it. Now I see. Thanks you guys, you’re good people

It refers to either a limited time reward structure where you pay for the ability to earn rewards such as cosmetics or other perks in multiplayer games such Fortnite and Valorant (If you don't earn the needed XP within the designated timeframe then you may not even get those rewards you paid for) or access to DLC content in general for both single and multiplayer games where some functionality/maps/modes are gated behind a single or recurring payment.

A game will release a bunch of DLC that is bundled together or due to come out at the same time.
"Well sell you all of the DLC that came out 1 year after the game. Other releases are on you"

I will provide a point of view of someone who is a bit more pro "live service game" concept.
While I agree these can be scummy it depends on the game. I see them as evolution of the subscription model on which most MMORPGs functioned back in the day (and few big ones and several niche ones still do). You used to pay monthly sub to be able to play the game, now you have the opportunity to pay for the season pass. Depending on specific game you get content for free (no neex to pay the sub to access the content) and alonv with the content they will launch a season pass which gives you some goodies for playing. Gives them engagement numbers and expectable revenue stream.
In the above model, I usually decide whether I like the content that they bring and if I would realistically play it enough to get majority ofnthe rewards from the pass (because, and this isnthe worst part - once the season ends in 99% of the cases the pass goes away and its rewards are gone forever if you didn't have time to earn them). Then there are also games like Destiny where the content is not free but tied to the pass, so you basically purchase x months of content (+the rewards from the pass).
There are many shades of season passes - some could be considered fair, some are alright, some are ignorable and some are bad. All of them are made to make money - which in a live service game is I think fair, as they need to fund the ongoing developement. It all depends on the way they want to takenyour money: can be used as a nice bonus you can purchase if you are enjoying the game - in which case I'm fine with it. It can be used so they want to manipulate you through various way to feel forced to purchase - in which case I'm very much oposed to it.
Though I think we all agree that the part when the rewards you purchased are only available temporarily is a scam and shouldn't be a thing - and I do realize I will be called out here for sometimes purchasing them. I try to view them as simply making a voluntary payment for the content I got from the devs. And the rewards tied to it a bonus.

A way to overcharge money for usually shitty DLCs that release on a schedule post main game launch. It's a marketing trick to get you to pay upfront for yet unreleased extra content, with the incentive that you can save a measly $5 or something. Often times these are just cap cosmetics. Once in a while it's a good DLC but usually not.

Can only explain it in examples.
Example: Fortnite, call of duty
Free to play. (Of you laid for the game)
You have a list of items that you unlock through progression/leveling up.
Season pass: a second line of extra stuff you unlock simultaneously leveling up/through progression.
Season pass stuff is usually the cooler stuff. If you don't care about cosmetics then season passes are useless.