Description from the Dor Un Avathar
Monsters with this vulnerability may not attack unless provoked. This generally means an opponent must attempt to strike first. Provoked does not mean using lame excuses to join combat. Because of this vulnerability, playing such a Monster may get boring in terms of combat (all the more reason to role-play).
 Additional thoughts from the Celestial Codex
In the context of this volume, Beneficent Angelicals and Celestials may launch attacks against Demonic or Infernal enemies without any provocation being needed. The Heavenly Host offers nor expects any quarter from their eternal foes. Anti-Paladins, Darklords, Undead and other obviously evil players are included in the “kill ‘em all” category for Beneficent beings.