Google has a long history of 'incubating' apps and services in Beta form for longer than seems necessary. GMail itself was one of those apps that Google kept in Beta for years, and the same is true of Google Earth and other products and services. "Undo Send", a feature that has been in Beta for years on GMail recently got the gold seal on the web, and now it's arrived in Inbox, which is ironically powered by GMail and even developed by the GMail team. The feature works exactly as it does in GMail, except that this version of the feature is available in the Inbox app on Android.
Undo Send basically does what you think it does, when you send a message that you didn't mean to send, or it wasn't quite complete, you can hit the Undo button and it'll turn your message back into a draft again. This is of course pretty hand if you're prone to hitting enter or send a little too early, and the Inbox team have done a good job of highlighting why Undo Send would be helpful in the below video. The feature is live right now on GMail as well as Inbox, and the Inbox app on both Android and iOS, too.
These are the sort of features that make Google services unique and let's be honest, the ability to take back an email is something that a lot of people could really use. This is the sort of thing that's similar to the attachment checker that's been in GMail for years now, saving people from embarrassing double emails just get the attachment you said was attached through the first time. While many of us have moved on from email, there are a lot of people that still use older methods of communicating like SMS, and as many as 5 Billion people around the world are still reached using SMS messaging. Undo Send has finally broken out of beta and has moved over to Inbox as well, perhaps they can get started on an Android tablet app now?