In short: Google announced today that it is shutting down its Inbox email app in about six months, March 2019. The service will stop working in March - though no specific date in March has been announced. Google has already brought many features from Inbox over to Gmail, so one might say that it was inevitable that Google would kill off Inbox. However, there is a pretty big following for Inbox, so Google may face some backlash here.
Background: About four years ago, to the day, the Gmail team announced Inbox. It was an off-shoot email app that was developed by the Gmail team, and would feature a ton of great features that made it easier to get through your email each day. This included better sorting for email, as well as the ability to snooze emails to come back to your inbox tomorrow, next week or even a few months from now. Inbox made it easier to deal with all of the email that we get every single day. What started out as an invite-only service, turned into a pretty popular email app.
The impact: This means that for those that use Inbox for their email, they'll need to look elsewhere to deal with email. Whether that is going back to the Gmail app, or using a third-party app/service. Many will likely go back to using the Gmail app, which did get a nice Material Design makeover a few months ago, and has also incorporated a number of Inbox's features already. So it's not the end of the world. These types of things happen with Google quite regularly. Google will unveil a number of services and/or products, but not all of them will remain long-term. Back in 2012, Google announced it was killing off Google Reader in early 2013 as part of its "Spring Cleaning" and there were all kinds of petitions being signed and so forth, but that didn't keep Google Reader from being killed off.