Astral Barrier is a shield that blocks damage when in Astral Shift. In the earliest editions of the story was named “Aura”, but was changed to “Astral Barrier”, or just “Barrier” in later edits. Also known as a shield.
“Your Barrier is also adaptive,” Everglaive said. “Meaning it knows how much your Vitality and Spirit will naturally resist and let the appropriate amount of damage through before draining your shield. For example, as a Guardian who focuses on Vitality and has over 900 in the stat, my physical defenses are quite high. As such, my Aura knows to let most of the damage ‘bleed’ through the shield to be negated by my body’s enhanced resilience instead of burning itself up against an attack.” Ever ch.35
“Your shield can’t always absorb all the damage. If an attack is strong enough or has some kind of piercing aspect, it can get through much easier.” Naiad ch.35
Mai can see her Barrier's remaining percentage in her HUD. Other MGs (Cutie Pie in ch. 25) can get an exact figure by looking at their status sheet, but if they don't do that, can only estimate by looking at how intact the clothing (Astral Dress) of their Astral Shift is.
How Stats affect Aura:
“Willpower determines your Aura’s overall capacity, Vitality gives your Aura better defense against physical attacks, and Spirit does the same but for magical damage. Please note that Vitality and Spirit do not provide nearly as much damage reduction to your shield as they do to your body. Spirit also determines how quickly your Aura regenerates. Aura regeneration is generally slow, taking sixteen minutes and forty seconds if your Willpower and Spirit are of equal value” Selene ch.35