Basically, for each heal over time you have on a target, your mastery boosts your healing done to them by your mastery percentage. If you have 15% mastery, and three hots on the target, then it increases the healing on that target by 45% (3 x 15%).
Mastery: Harmony
Requires: Level 78
Your healing is increased by 7.5% for each of your Restoration heal over time effects on the target.