The Stagger Meter in Diablo 4 is designed specifically for the larger Boss fights in the game.
At the top of the screen during a boss fight, there is a red bar showing the health of the boss as combat progresses.
Combat Milestones
The bar is divided into sections indicated by the triangles. These triangles indicate the progress of the fight and are milestones in the battle.
When they are reached, the boss may slump or stagger giving players time to get some extra unopposed hits in before the next stage of the battle.
It's worth noting that when a triangle milestone is reached, the boss's behaviour and telegraphs are likely to change for the next stage of the battle.
Health Drops
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.
Finishing off the Boss
When the meter reaches zero, the boss will die and victory is secured.