By default when you send a reply to an email the original message (or conversation if it is a multi email chain) stays in your Inbox until you remove it. With the Send+Archive button you can automatically archive the conversation when you send the reply, which takes it out of your inbox and lets you focus on items that have not been addressed. Please note: if your reply receives a response the entire conversation, along with the new email, will return to your Inbox.
This feature must be activated in Settings by clicking on “Show Send & Archive Button in Reply.”