Recoil is the State Management Library for React they use at Facebook.
Recoil lets you create a data-flow graph that flows from atoms (shared state) through selectors (pure functions) and down into your React components. Atoms are units of state that components can subscribe to. Selectors transform this state either synchronously or asynchronously
Best to watch the talk from React Europe 2020 embedded above. It clearly explains why Recoil come to exist, and when & how to use it.
If you’ve ever used MobX it might all sound a bit familiar. This Twitter Thread by the MobX author is worth a quick read.