A communication that is end-to-end encrypted guarantees the security of the data you send and receive from the time it leaves your device until it reaches the recipient's.
On your smartphone, launch the Facebook Messenger application. From the bottom menu, select the Chats option. The Edit icon, which looks like a pen, is located in the top right corner of your screen. Tap it. The lock icon's toggle should be turned on at the top right. Next, choose the chat partner's name from the list.
When you take steps like clearing your browser's cookies or browsing in private mode, end-to-end encrypted messages may be removed from your browser's storage, making it possible that you won't be able to access them in the Messenger chat on that browser again.