Hard fun is, of course, the idea that we take pleasure in accomplishing something difficult: the joy in meeting and mastering a challenge. As a result, when someone is doing something that is hard fun, moment by moment it looks more like "work" than "fun," but the net effect is pleasurable overall.
Simply put: Epistemic games recreate in game form the things that people do in the real world to learn to think in innovative and creative ways about problems that matter.