Favour is a Prowess, this special kind of Feat can't be acquired unless the character has Divine Touch. A Prowess is a special action that can only be done once per scenario. Like any Feat, it can be acquired only once.
Somebody powerful owes you a favour or you are yourself quite powerful and have a large network ready to help you.
When gaining this Prowess, choose five Feats (not Prowesses or Archetypes).
When using this Prowess, you can bring in your team one person with one of the five feats you have chosen to help you for one task.
The person needs few days to arrive and will only help you for one task.