Ever sent an email to someone you didn't want to send to? Perhaps you started to second guess yourself after sending an email with questionable content...
Before, there was no way to reverse an email after you clicked "send." Now there is. There is an "undo send" option in Gmail...but you have to enable it.
The catch is that you'll only have a couple seconds after the email has been sent to click the "undo" option at the top of the window. Also, this is an "experiment," so it could disappear at any time. It might just come in handy though!