Tracks defensive buffs on you, so you can utilize them better and don't overlap their usage. It just shows a simple progress bar, nothing special.
What it tracks:
Death Knight - IBF, AMS, Vampiric Blood, Unbreakable Armor, Sigil of Unfaltering Knight, Sigil of Deflection, Blade Barrier (talent)
Paladin - Holy Shield (with stacks), Sacred Shield, Divine Guardian/Sacrifice, Divine Protection, Hand of Salvation
Warrior - Shield Block, Shield Wall, Last Stand, Spell Reflection
Druid - Barkskin, Frenzied Regeneration, Survival Instincts
External Cooldowns - Pain Suppression, Guardian Spirit, Hand of Sacrifice, Mark of Blood (on your target)
Trinkets - Figurine - Monarch Crab, Royal Seal of King Llane, Heart of Iron, Furnace Stone
Potions - Indestructible Potion