The Archive Chat feature allows you to hide a conversation from your Chats screen and access it later, if needed. You can archive group or individual chats to better organize your conversations.
Note: Using Archive Chat does not delete the chat, nor does it back up the chat to iCloud. Archive Chat is only available for devices with iOS 7 or later.
You can archive all chats at once in WhatsApp Settings > Chats > Archive All Chats.
Archived chats will unarchive when you receive a new message from that chat. You can manually unarchive a chat by searching for a contact name or message from that contact:
Alternatively:
Learn how to use Archive Chat on: Android | Windows Phone | WhatsApp Web