Wizards Unite is the spiritual child of Pokemon Go, offering a similar style of exploratory AR gameplay, involving discovery and collection, and bringing together fans across the globe.
Adventure sync allows you to register the distance you have travelled even when you don't have the game open. In both games - Pokemon Go and Wizards Unite - the distance you walk is a gameplay element, allowing you to hatch eggs in Pokemon or unlock portkeys in the world of Harry Potter.
Exactly why this wasn't included in Harry Potter: Wizards Unite from the off we don't know, as being able to have your phone register how far you have walked and have that distance credited in the game is a huge benefit - especially in events where unlocking new portkeys is essential - like the recent Global Challenge.
Niantic hasn't actually confirmed when this feature will appear, but it was included in a recent newsletter sent out to those on the Wizards Unite mailing list. It was buried towards the bottom of the page, under information about the other events happening in the Wizards Unite universe.
Once this is available, all you basically have to do is enable adventure sync - which gives the game access to your phone's motion tracking stats and you'll be unlocking those portkeys whenever you're walking.