World Tiers in Outriders are a unique difficulty system. Instead of general easy, medium and hard options, Outriders employs an interesting alternative that incentivizes players to ‘earn’ higher difficulties and, in turn, get better gear.
Coming to terms with how this system works will define your time in Enoch, especially as you make your way through the campaign.
After that, there are Challenge Tiers and… well, it gets complicated. Ultimately, these are systems with a lot of wrinkles that you can game for more efficient progress or even better loot.