It was a pretty decent email app developed by Google back in the day. They discontinued it a few years ago and some features made it into the Gmail app, but not all.

Literally second paragraph of the article:

For context, Google set out to revolutionize how we deal with emails with its alternative Gmail interface Inbox in 2014. The service eschewed the chronological thread-based nature of regular email clients in favor of a to-do style interface, allowing you to reorder, snooze, and bundle emails. Inbox was killed just five years later, and we’ve been left without a proper in-house replacement ever since