Niantic and The Pokemon Company have rolled out the Adventure Sync feature to Level 40 trainers in Pokemon GO.
Adventure Sync is a new feature that allows trainers to rake up walking distance even when Pokemon GO is not running in the background. Walking distance tracked through Adventure Sync is used to for hatching eggs and earning items.
Niantic plans to roll out Adventure Sync gradually to more trainers on a level-by-level basis. Eventually, it should be available to everyone who’s at least Level 5. We believe this will take a few weeks based on past experience.