So what's so great about "undo send"? Gmail doesn't send email immediately and there's a short delay that lets you undo sending the message, just in case you changed your mind, you noticed a mistake or you picked the wrong sender. There are 4 options for the cancellation period: 5 seconds, 10 seconds, 20 seconds and 30 seconds.
After sending a message, you'll get an "Undo" link next to "Your message has been sent". If you click the "Undo" link, you can edit the message, save it as a draft or delete it.
"Undo send" has been recently added to Google Inbox for desktop and mobile. Mashable reports that Gmail's mobile apps will also add this feature: "Google said it will roll out the feature to mobile in the future but didn't elaborate on the timeline."