To archive a conversation, right-click on it on the conversation lis and choose Archive from the drop-down menu that appears.
This action will put the conversation under the Archived label, at the end of your conversations list.
To Unarchive a conversation, right-click on the archived conversation under the Archived label and choose Unarchive from the drop-down menu that appears.
After you Unarchive a conversation, it will re-appear under Recent on the conversation list.
Do note that any message posted in an archived conversation will also automatically unarchive it.