When you heal a target with your Heal, Greater Heal or Flash Heal spell, the duration of the Weakened Soul debuff on the target is reduced by 2 sec.
Strength of Soul is a passive priest ability learned at level 52 for those with the Discipline specialization. It decreases the duration of the Weakened Soul debuff ([Power Word: Shield]) by two seconds when healing the target with [Heal], [Greater Heal], or [Flash Heal].
Patch changes
Patch 4.1.0 (26-Apr-2011): It is now possible to remove Weakened Soul effects that were a result of another priest's Power Word: Shield through Strength of Soul.
Patch 4.0.6 (8-Feb-2011): Strength of Soul now occurs when the priest casts Inner Focus on oneself, rather than Power Word: Shield. In addition, Strength of Soul now also causes the priest to become immune to silence, interrupt, and dispel effects for 2/4 seconds after using Inner Focus.