Battlepasses are usually an another progression system with time limited skins and rewards. Usually a season last a couple of months, then a new one starts with new progression and rewards. Sometimes there are events tied to a season.
In theory, it means the game always gets updates, but in actuality its ripping off players for like a handful of skins (theres a lot of filler in most of them), while getting them to play the game more because of fomo.
Also sometimes you can earn back the price back in in-game currency, so if you keep maxing them out its “free” after the first one.