You gain 12% of your total mana over 9 sec, but the amount healed by your healing spells is reduced by 50%.
Divine Plea is a paladin ability learned at level 46 for those with the Holy specialization. It restores 12% of a paladin's mana over nine seconds, but reduces healing output during that time. This is mainly used by Tankadins in order to restore mana after an unexpected mana-burn, for example. Paladins will not use this very much during healing intense fights due the massive healing reduction.