In context, this quote from Mere Christianity probably refers to a soul yearning for God and His kingdom. (I say “probably” because I haven’t actually had the pleasure of reading that one yet. Seriously recommend Screwtape Letters though. I digress.)
However, I also think it refers to creative people, yearning for things that don’t exist in this world, and creating them.
Or, you know, it might mean psychopaths who traipse around town in their everyday clothes with regal robes and gold crowns thrown on. I’m leaning toward the heart of creative people personally, but hey, if you like that interpretation best, whatever paddles your canoe.