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.
Notes
The amount of mana gained is based on your total mana, so more intellect will mean more mana is restored.
Patch changes
Patch 3.1.0 (14-Apr-2009): Divine Plea can once again be dispelled.
Patch 3.0.9 (2009-02-10): The amount healed by your spells is reduced by 50% (up from 20%), but the effect can no longer be dispelled.