Guard is a defensive Buff in Path of Exile 2.

Note for Path of Exile 1 players:
Guard is a Buff in Path of Exile 2 instead of a Skill type. Guard is functionally similar to some Guard Skills from Path of Exile 1.
Guard is a defensive Buff in Path of Exile 2.

Note for Path of Exile 1 players:
Guard is a Buff in Path of Exile 2 instead of a Skill type. Guard is functionally similar to some Guard Skills from Path of Exile 1.
Guard is a temporary Buff that absorbs Damage from incoming Hits. The Damage that is absorbed this way is removed from the Guard Buff counter before Life or Energy Shield.
Note: Guard only works for incoming Hit Damage. It does not interact with incoming Damage over time.
A Guard Buff will absorb Damage up to a maximum amount before being fully depleted. Upon depletion, any remaining Damage that wasn't absorbed is dealt to the next resource, usually Life or Energy Shield. You can only have one Guard Buff active at any one time.
Guard is considered to be a Damage Absorption effect, but not all Damage Absorption effects are Guard Buffs. An example of a Damage Absorption effect that is very similar to Guard, but is not a Guard Buff, is the Witchhunter's
Sorcery Ward.
The Duration of a Guard Buff is determined by the source of the Buff. Skills such as
Fortifying Cry provide a Guard Buff with a 4 second Duration. However this Skill also notably has a Duration tag, allowing the Duration of the Buff to be extended by modifiers that improve Skill Effect Duration.
Note: Guard Buffs obtained from non-Skill sources are not affected by modifiers to Skill Effect Duration.
The total amount of Damage a Guard Buff can absorb, known as its Magnitude, is determined by the modifier that created the Buff.
For example
Olroth's Resolve is a Life Flask that provides instant Life Recovery. When this Flask provides Recovery that is considered to be excess Recovery, such as when on Full Unreserved Life, this portion is gained as a Guard Buff. Another example is
Fortifying Cry which grants a Guard Buff based on the total Enemy Power in the Warcry radius.
Importantly, when you gain a new Guard Buff whilst you already have an existing Guard Buff, it is not simply replaced. Instead, the Guard Buff with the higher Magnitude will take priority, whilst the Duration of the resulting Buff is proportionally modified by the Magnitude of the discarded Buff. The modified Duration cannot exceed the original base Duration of the Buff.
The Warbringer's
Jade Heritage Ascendancy Notable passively generates stacks of Jade and grants the
Encase in Jade Skill. This Skill can be used to consume all of your Jade stacks, grating a Guard Buff based on your maximum Life, scaled by the amount of Jade stacks consumed. Given the Life scaling, this Skill can provide access to one of the largest Magnitude Guard Buffs in the game.