One of the new features with 4.0.1 will be the option of reforging stats on pieces of armor.
The quick answer is you reforge 40% of a secondary stat (parry,dodge,haste,hit,crit,etc) into an equal part of a different secondary stat.
Let's explore this a bit more.
40% of 70 Dodge rating is 28, so 28 is the number that is dropped and reforged into a new stat.
The cost of reforging was 10g or what the item is worth to vendor, whichever is higher.
Only one stat can be reforged on the item and you can drop a reforged item back into the window and instead of Reforge you can click Restore to restore the item to the original stats. Once you reforge an item a green text will appear under the item name saying, "Reforged."