Equilibrium Level is a game system in Honkai: Star Rail that raises the difficulty of the game world by increasing the average level of enemies and encounters. It is essentially the world level of the game, which determines the difficulty of the enemies and the rewards you get from their drops. Certain tasks, upgrades, and more rely on your Equilibrium Level, and you cannot ascend your characters or light cones past a certain threshold unless you have increased your Equilibrium Level.

To increase your Equilibrium Level, you need to complete the Trial of the Equilibrium rift when you reach certain Trailblazer levels. Your Trailblaze Level determines your Equilibrium Level in Honkai: Star Rail, and you can check this from your profile. As you reach specific level thresholds, you will be granted a quest to raise your Equilibrium Level. Equilibrium levels give you more loot in Calyx and other dungeons, and not raising it will make it harder for you in the long run.

In summary, Equilibrium Level is a game system in Honkai: Star Rail that determines the difficulty of the enemies and the rewards you get from their drops. You need to complete the Trial of the Equilibrium rift to increase your Equilibrium Level, and certain tasks and upgrades rely on it.