The Knight Bus is one of the most-loved parts of Harry Potter and the Prisoner of Azkaban, a wonderfully amusing solution to get Harry Potter through the streets of London in frenetic style. It's also the solution to a particular problem in Harry Potter Wizards Unite.
Niantic, best known for its exploration games, has been responding to people staying at home and the Knight Bus is a great example of that. As you can't go outside to complete some of the tasks needed in Wizards Unite, the Knight Bus means you can keep playing some elements of the game.
The icon for the Knight Bus now appears in the top right of your game and tapping on it will see you jump on the bus to be transported to the fortress at Hogwarts Castle. Even though you can't walk to fortresses, you'll still be able to go and battle, thanks to this clever little addition.
Harry Potter Wizards Unite already has some elements in it that allow you to get out in the virtual world, using Portkeys. Sadly, these normally need you to be walking to unlock them, but thanks to the recent addition of Adventure Sync to Wizards Unite, you don't have to actually have the game open for that to happen.
That means that as long as you have your phone with you, you'll still be able to get any steps you take to count in the game - that might be running around the garden, your allotted exercise time, going to the shops or walking the dog.
The Knight Bus, admittedly is a clever solution to a unique situation - the wonders of magic coming through in Harry Potter fan's time of need.