Reply and archive moves a conversation out of your Inbox and into Archive as soon as you send your reply. It's off until you turn it on.
Steps
Desktop
Open Settings: press ⌘, on Mac, or Ctrl+, on Windows and Linux.
Go to the General tab.
Under Reply Behavior, switch on Reply and archive.
iPhone, iPad & Android
Open the menu and tap the Settings gear at the bottom.
Tap General.
Under Reply, switch on Reply and archive.
To do it for just one reply, without the setting, touch and hold Send and tap Send & Archive.
Good to know
You'll only see the setting while Group emails into conversations is on. It's in the same place in Settings.
It works on replies and forwards in an existing conversation. A brand-new email isn't affected.
On desktop, the conversation is archived once your email has been sent; press Z to bring it back. On your phone, it's archived as soon as you tap Send, and undoing the send puts it back in your Inbox.