Every class can learn an ability called "Mastery" from their trainer at L. 75.
Depending on which spec you choose to put 31 points in, their "Mastery" will do the things mentioned on the Talent Summary page which you were looking at. e.g: a frost mage will deal increased damage against frozen targets.
When Cata is released, there will be a "Mastery" stat rating on some items, which will increase the power of the "Mastery" ability. If you spec to fire for example, and keep the same gear, your "Mastery" will be at the same rating. But it will do different things now, since you specced to fire: increase your fire dot damage.
You can now change 30% of any stat rating on an item to a stat which wasn't on the item before for 10g. So if you want, you can get some mastery rating right now by reforging crit or haste or something like that to mastery.