The Stagger Meter in Diablo 4 is designed specifically for the larger Boss fights in the game. This should not be confused with the Sorcerer's skill Stagger.
At the top of the screen during a boss fight, there is a red bar that reflects the health of the boss as combat progresses. Underneath is a thinner bar which slowly fills as you attack the boss with skills that have some a Crowd Control element. Once completely filled it turns blue and begins to drain pretty quickly. During this short time the boss is 'Staggered' meaning it is under the influence of all forms of Crowd Control at the same time. These are:
- Knock Down
- Knock Back
- Pull In
- The monster can not attack or move
- Any skills you have that take advantage of Crowd Controlled monsters will be particularly effective.
Some examples of that would be the Barbarian's Enhanced Double Swing which gains extra Fury if the target is Stunned or Knocked Down or his No Mercy passive that deals increased damage against Immobilized, Stunned, or Slowed enemies. The Rogue on the other hand has the passive Frigid Finesse which deals increased damage to Chilled enemies. She also has Primary Invigorating Strike which increases Lucky Hit Chance if the target is Crowd Controlled.
With this in mind a specific build that takes full advantage of these precious seconds when the Boss is Staggered will be advantageous.
When one of these new stages of battle are reached on the Stagger Meter, the boss is also likely to drop health potions. This allows players to time the use of potions more carefully during the combat.