Any Gmail user who regrets a message they just sent has the option to undo the action, provided they're quick enough. The new Undo Send option allows a message to be retrieved, so long as it's clicked upon within 30 seconds of the Send command being hit. The idea is to give users a second chance if they catch a mistake or realize they've sent a note to the wrong person.
The function has, so far, only been available as an experimental feature. It is still not automatic: availability has to be activated in the settings. Users can opt for the undo function to work for between 5 and 30 seconds after the initial send.