You have a X% chance when you Smite, Heal, Flash Heal, Binding Heal or Greater Heal to cause your next Flash Heal to be instant cast and cost no mana.
Surge of Light is a Priest Talent giving critical strikes a chance to remove the mana cost and cast time of the next Flash Heal.
Using with Flash Heal
While note above still actual for "Surge healing", there is at least one more thing to take into consideration. You may wish to increase your mana efficiency with these mana-free casts, but doing that requires you to make additional instant cast and thus, use extra Global cooldown. As Flash Heal is minor (that is the main difference from "Surge Smiting", where you will cast the same Smite spell, not some minor variation) healing spell and provides relatively little heal, "Surge healing" may sometimes reduce your HpS. On other hand, you will often have some kind of instant-cast "emergency spell", if you get enough critical heals to keep Surge of Light buff up for most of the time. This instant-cast heal may be used when your common Power Word: Shield is unavailable, for example, due to Weakened Soul debuff. Consider these notes before deciding, if you want to spend your talent points in this way or not. This talent is very useful in pvp cause its instant cast ability could make the difference between life or death.