You can clearly see on the image above on what’s the concept behind this new feature “Send & Archive” is. Soon after replying to any mails, most of the Gmail users have a habit of archiving mails instead of deleting them. As such, this new button (as seen on the image) will help you to archive mails as soon you reply to them. All you need to do is use ‘Send & Archive’ button instead of just ‘Send’.
To activate this feature, go to Settings->Labs->Send & Archive and enable it.
(Via Lifehacker)