Extension
Extension is an advanced function for Messenger platform.
When a button’s hyperlink is connected with a website, you can select the prompt action to be an “extension”. Then when a customer clicks on the button, a website will be displayed directly on the current tab as an extension. This will not open any new windows or new tabs.
When using “Extension” function, the linked website only needs support for mobile devices. There’s no need for an extra desktop version of the website.
Currently, websites need to meet the standards of the Messenger Extension SDK in order to be used as extension windows.
For more detail and official documentation, visit https://developers.facebook.com/docs/messenger-platform/guides/chat-extensions