In hard worldbuilding the creators often explain everything from how the universe works to technology and culture, to justify the world, the story and the gameplay, so the player knows what to expect. They use specific rules, consistency, transparency, details and realism to create the immersion. Everything needs to make sense within the logic of the world.
Sometimes the creators dig themselves into a hole when trying to explain the background story. You see this when a sci-fi story might get drawn out because of the need to explain every detail for the world to make sense. That can translate into way too long intro-sequences, cinematics or tutorials.
Good game design can build the world understanding into the game flow, by letting everything slowly unfold and it can become a great feature and help immerse the players as they are eager to learn more. As long as the unfolding of events follows the logic of the world, there’s no problem, but the more content the creators add, the more they must keep track of to make sure it all fits together.
An example of a game with hard worldbuilding is Red Dead Redemption by Rockstar Games. It’s full-on realism based on the cowboy tropes we know and love, so even though the gameplay is interesting, you are never surprised by the actions you do or how the world looks and feels. You can find a number of side-activities to do, all realistic and fitting within a cowboy story