Below a hostile’s health gauge, there is a secondary gauge that shows the amount of “Stagger Damage” it has taken. When this gauge is full, the enemy will be completely immobilized for a short period of time, during which it will be extra susceptible to damage.
You can recognize a successful Stagger, because the hostile will glow red and havered sparks around it.
However, not all weapons in the game deal Stagger Damage.