I'm still not clear on what battle passes really are. To me, "Season Pass" just means you get all the DLC that shows up over the next year (or whatever they decide to call a season). It doesn't have anything to do with whatever freaky currencies a game has.
But then people talk about having to grind for them... huh?? Is this something about those "daily quests" things? I've only seen those in Assassin's Creed: Odyssey and I think one of the Watch Dogs games - and I barely gave them a second look.
It is related to those "daily quest" things. Games with daily quests don't necessarily have a battle pass and games with a battle pass don't necessarily have daily quests, but they both serve the purpose of getting the player to keep playing the game regularly.
A battle pass is a system where a player can earn points by playing the game (for example, by completing daily quests) and get a reward every time they've earned enough points. Notably, a battle pass is (typically) only valid for a specific amount of time, giving players a time limit if they want to earn every reward on the battle pass and giving them an incentive to keep returning to the game.
In some (most?) games you have to pay to receive the rewards from the battle pass. Often, some of the rewards are free, while you have to pay to get the rest of the rewards. Of course, every time you claim the free rewards the game also shows you all of the rewards you would get if you pay for the battle pass.