https://www.gamespot.com/articles/pokemon-go-adding-mew-and-quests-in-big-update-thi/1100-6457687/

A big update is on the way very soon to Pokemon Go. As first discovered last month, developer Niantic is implementing a quest system in Pokemon Go, which will give players a variety of additional activities to complete--as well as a chance to discover the game's first Mythical Pokemon.

Beginning March 30, players will be able to take on two different types of "research" quests: Field Research and Special Research. The former are distributed via PokeStops and task players with completing various objectives, such as catching certain Pokemon or participating in battles; the latter are "story-driven activities" assigned by Professor Willow that "take Trainers further into the world of Pokemon Go than ever before."