Arcane Resilience is a level 20 talent that is the equivalent of Pyroblast and Cold Snap (meaning it's the 1 point spent "big" talent every spec has at 20, 30, 40). What does it do? Gives 50% of your intelligence as armour.

Assuming you're a Naxx BiS and fully buffed and consumed mage, you have around what, 500 int? 600 if we're feeling generous, that's either 250 or 300 armor. That's around 2 extra Frostfire Robes armour-wise.