Reforging allows you to pick up an item that has a secondary stat that is undesirable to you, and turn it into something more desirable.

You can take 40% of a stat that exists on the item, and turn it into the equivalent points of a stat that is not already on the item. So for example, if I have an item with 100 Hit and no Spirit, I can reforge 40% of the Hit (40 Hit) and change it to Spirit, giving me an item with 60 Hit and 40 Spirit, because I took 40 of the Hit and turned it into Spirit.

So you can take part of the less desirable stat and turn it into something more desirable, but you cannot strip it off entirely. Still, this helps a lot when you perhaps pick up a DPS item or an item with no regen, and can turn it into something better suited to healing.

For more information on Reforging, visit Wowpedia’s page on Reforging.

How should I reforge?

At low levels of gear, for example when you are starting in heroics, you will likely find that mana is a problem for you, and your haste will be very low. If this is the case, you will need to reforge to Spirit on non-Spirit items, and Haste otherwise, where possible.

At higher levels of gear and for raiding, once you feel comfortable with your mana, your rotation, and your regen, and if you have reached the appropriate haste breakpoints, then you can start moving away from reforging to Spirit and Haste and lean more towards balancing your stats, and allowing Mastery back in. Remembering though that Spirit is still essential, so think carefully before reforging any Spirit *off* your gear.