Facebook Messenger’s Secret Conversation is a feature that enables users to have a private, encrypted, and protected conversation.
One user can activate the feature on the app and start a secret conversation with another user. Since then, the texts, pictures, files, and stickers in this conversation are only available for these two users. And whoever sends them has the right to set a timer for the messages to disappear forever.
Moreover, if one person switches to another phone or tablet to use Facebook Messenger, the other person will be notified of this change.