ReforgeStats is a very simple addon designed to help the reforging process.
Personally when I'm reforging I'm usually trying to get back to the hit rating cap after changing an item, the problem is that the reforging UI simply says that you can have -30 crit rating here or +40 hit rating there, and it can be difficult to figure out which item would be best to reforge because it's hard to tell just how much hit you're going to gain, or how much crit you're going to lose.
ReforgeStats calculates the effective change that would result from a particular reforge and displays it in the reforging UI allowing you to see just how much hit you're going to get and whether that +40 hit rating is going to get you to where you want to be.
That's it, it's that simple.
Note: Mastery and Expertise get a little more complicated, ReforgeStats can work out how many 'Mastery Points' you will get from a particular Mastery Rating reforge, and from there your particular talent spec dictates how your Mastery Points translate into physical results. As an example, if i were to reforge an item on my Arcane Mage to get +68 Mastery Rating, ReforgeStats tells me this equates to 0.38 Mastery Points. From my talent spec I can see that each mastery point increases my damage done by up to 1.5% so the 0.38 Mastery Points equates to an extra 0.57% damage though ReforgeStats can't calculate that itself. Similarly ReforgeStats can calculate that 68 Expertise rating equates to 2.26 Expertise, but can't calculate how that equates to Dodge/Parry chance.