Have you ever sent an email and then immediately regretted it or realised you made a mistake in the content? I know of someone who ended up in a nasty defamation case because of that – they replied to the wrong person with an imprudent email.
Well Gmail is making Undo Send a standard feature. What it actually does is delay the message send for up to 30 seconds to give you a chance to change your mind. You determine the time in your general settings. Kind of a cooling off period.
Following a general rule of email etiquette will help you as well and mean that you won’t need this feature. Never write anything in an email that would embarrass you if it ended up in the wrong person’s mailbox. Emails frequently get forwarded to a number of other people so can easily go somewhere you were not expecting them to end up. Always assume that that could happen and write emails accordingly.