Stagger is a penalty taken from being hit in the head, balls, or otherwise sensitive locations, as well as from being caught in blast effects etc.
It penalizes most rolls.
At the start of a character’s turn their stagger penalty is reduced by their dots in Stamina.
Healing effects remove stagger. Healing which would usually remove a level of bashing may instead remove stagger penalty equal to the character’s Stamina.
A character can also use the Take Stock maneuver to reduce stagger.
If stagger penalty would reduce the roll for an action to 0 the action may not be attempted, even if no roll would usually be required (story teller decides which attributes and skills apply in that case).