Divine Plea is a paladin ability trained at level 71. It restores 5% of a paladin's total mana every 3 seconds for 15 seconds, but reduces your healing output during that time. This is mainly used by Tankadins in order to restore mana (and reduce damage taken), after an unexpected mana-burn, for example. Holy paladins will not use this very much during healing intense fights due the massive healing reduction.
The amount of mana gained is based on your total mana, so more intellect will mean more mana is restored.