In Genshin Impact, the difficulty of enemies, bosses, and the quality of the rewards they drop, is determined by world level. But as this scales with adventure rank, and not the actual level of your characters, difficulty can sometimes catch you off-guard. If you’re finding things tough, you can lower world level by one in the menu overview. This will make enemies and bosses easier, but be warned, it will also decrease the level of potential item drops you get from enemies.
There are eight world levels overall, but for adventure rank 25, 35, 45, and 50, you’ll also have to complete a quest to raise it, meaning that you can personally choose when you’re ready for things to get more difficult. World level also determines how high you can level your characters, so when the game tells you the level cap has been reached, you know its time to upgrade, or earn adventure rank until you’re ready to upgrade.