What do these yellow outlines on health bars mean?

What do they actually mean? Seems not to be regular energy shield. Usually can be seen on Expedition spawned mobs.

Ward?

Yea that's Ward

Thanks! Now it's clear.

For those who also curious here is a quote from wiki:

Ward is a defence type introduced in content patch 3.15 that absorbs the damage taken from a hit equal by the amount of ward. After being hit by any amount of damage, ward will deplete, recharging after 5 seconds. Unlike energy shield, taking damage will not interrupt ward restoration time.

Ward can absorbs all type Damage. Physical damage,Fire Damage,Cold Damage,Lightning Damage,Chaos Damage.

Ward can only be found in item bases found from expeditions and Iron Flasks.