How many times have you hit "Send" on an email, only to realize you goofed. More often than you want to admit, right? Outlook has a recall feature that works on Exchange ... some of the time. But basically emails sent via the Internet, once you hit "Send," they're gone. Until Gmail's "Undo Send" feature, that is.
It's another Gmail Labs feature, so to enable it you have to go into Settings, Labs, scroll down to Undo Send and enable it.
It's the same thing as sending any other email via the Internet, though. Once it's gone, it's gone. What "Undo Send" does is hold your email for 5 seconds before sending it.
During that time, if you click the "Undo Send" link, it will not only stop the email, it will roll back the email to the point you were at in composing it before you hit "Send."
Why only 5 seconds? They didn't want to add significant lag to the emailing process. You know how people complain when they don't get something "right now." Still, they are taking feedback, and will obviously tweak it as necessary.
Will it save you? I tested it and it worked. Don't try to send yourself an email to test it, though. In that case the email still showed up. Sending an email to my wife, and hitting "Undo Send" worked great, however.
This isn't the first Gmail Labs feature set to save you from yourself. Google earlier introduced Mail Goggles which would keep you from sending an email if you were too drunk or groggy to answer a math question.