Your Ritual Fatigue attribute is increased by +1 for each level of this ability that you possess.
If you are a Cleric, each time you take a level of this Ability you master two Ritual Lists of your choice that are granted by your chosen deity. You cannot ever master the Astral Codex.
If you are an Arcanist, each time you take a level of this Ability you master two Ritual Lists of your choice. You cannot ever master the Celestial Codex.
See the Ritual Lists page for the available lists you can choose from.