Details on the game are pretty scarce, so it's not clear what the game will involve or what its mechanics will be. Niantic did say in September that the company was looking at using audio cues in future AR games because holding up a phone for an extended period of time "makes them look like a total doofus," according to Niantic CTO Phil Keslin. It's possible that Harry Potter: Wizards Unite will use sounds as cues.
According to TechCrunch, the game will borrow from Ingress, a previous endeavor from Niantic Labs. Players will move through their physical world to collect power-ups and defend locations (presumably, similar to Pokémon Go's gym system). Whatever the mechanics of the game, with the Harry Potter franchise extremely large fanbase, it's safe to say there will be a lot of interest in this game.
Update, 4:30PM ET: Since this story was published, Niantic released more details on Harry Potter: Wizards Unite. Unfortunately, there aren't any details about when the game will arrive. As expected, Niantic says the game will put players into the Harry Potter universe through augmented reality: "Players will learn spells, explore their real world neighborhoods and cities to discover & fight legendary beasts and team up with others to take down powerful enemies." We'll have to be patient for now to see how this all works, though -- there aren't any other details on gameplay to be had right now.