Let us say you typed your mail and hit the 'Send' button marking the wrong person in the mail. Gone is Gone! But now with a grace period of 5 seconds to rectify your BooBoo, it might well be just the right thing to save an Oops!
Google says the
Undo feature will give yourself a grace period of a few seconds to cancel sending, then edit your message before sending again. This feature can't pull back an email that's already gone; it just holds your message for five seconds so you have a chance to hit the panic button. And don't worry – if you close Gmail or your browser crashes in those few seconds, we'll still send your message.
To enable this feature, Open your Gmail > On the Top right Corner click 'Settings' > Click on the 'Labs' tab and find the 'Undo Send' feature. Enable it.
You can check out some other cool new features in What's New in Gmail?. Gmail is kind!