Vanish mode is a rather unique and new feature recently integrated into Facebook Messenger; this mode essentially allows a user to send temporary messages to other people who will disappear once they have left after the other person sees it, leaving absolutely no trace.
With Vanish mode, you can send any message, such as GIFS, videos, text, photos, etc. The messages will vanish once the other person on the receiving end of the chat sees the message, and you leave the chat after that.
Although it is still possible to access the messages for a few more hours after leaving the chat, you won’t be able to access it if you turn off Vanish mode on Facebook Messenger.
It is similar to the disappearing messages on the Snapchat app; you may also check out What is Vanish Mode on Instagram and How to Turn it On and Off.