Stagger is a negative combat condition in Valheim. An enemy is staggered after receiving enough knockback or lots of damage. A staggered enemy will lean back and flail their arms or a similar animation indicating they’re vulnerable. When staggered, all attacks deal bonus damage.
Player Stagger bar
The stagger bar is the bar that fills up to the right of your character each time your character is attacked. The more health you have, the more damage you can take before you are staggered. The value of health that matters is the maximum amount of health you could have. The stagger bar does not change depending on how much of your health is healed. If your stagger bar fills up all the way, you will be staggered making you more vulnerable to damage.
The amount of that the stagger bar fills up is only dependent on how much damage you have taken. So your stagger bar will fill up less when you are blocking vs if you weren’t blocking, because you take less damage.