Basically what the title says. There’s always discussions about someone being force sensitive and how they could use the force or not sensitive to it at all, but has anyone ever been completely immune to it? Like if Vader was doing a force choke and the person was just like “why are you holding your hand up like that? What are you trying to do?”
They are a major plot point in the Thrawn books. They act as Thrawn’s ace in the hole to try and defeat Luke, and they are a precaution for the insane Jeedi master he has recruited.