Reforging is the mechanics which can drastically change how a piece of equipment work. Basically you remove 40% of an unwanted stat and gain those 40% as something you want.
There is a caveat. The piece of equipment can't already include the stat you want.
So, how do we reforge?
I'm assuming you're fixing up your best set of gear for the hardest content you're currently tanking.
Always reforge away from threat stats. Those would be hit-rating, expertise-rating, crit-rating and haste-rating. There is an exception to this. If the piece comes with an unholy bulkload of avoidance (dodge/parry) and a very meager amount of threat stat, then it might be better to reforge away from the avoidance stat. The break point should be about when the threat stat is 60% of the avoidance stat in raw numbers.
Always reforge into mastery. If it's impossible to reforge into mastery, then reforge into avoidance.
As for avoidance, you should aim at having around 1% more parry chance (not rating) than dodge chance after buffs.
While you can theoretically hit 102.4, if you're a casual tank reading The Pugadin, don't worry, you won't. At least not during this expansion.