Reforging allows you to modify the secondary stats on your gear. Use this guide to determine the best strategy for reforging your stats so that you can optimize the performance of your Restoration Druid.
Spirit > Intellect > Haste (Breakpoint) > Mastery > Crit > Haste (Past Breakpoint)
Scenario 1: Mana Issues
If you are having Mana issues during encounters, then you should reforge for more Spirit on your gear. On any gear without Spirit, reforge the weakest stat (see stat priority) into Spirit.
Scenario 2: Mana is Okay
If you are not having issues with Mana during encounters, you should reforge for more healing-output. This is best done by reforging lower priority stats (see priority) until you hit a Haste breakpoint. After this, reforge for Mastery.