MUFFIN is short for "My User Friendly & Flexible INterface." MUFFIN focuses on a "personal UI" depending upon a user's habits, is deemed user-friendly, and is flexible. These different UI elements will be available with the upcoming LibreOffice 5.3 and offer options for the default UI, a single toolbar UI, a sidebar with a single toolbar, and a new experimental "notebook bar" interface.
LibreOffice 5.3 is currently in beta with many changes throughout beyond just these new user-interface options while LO 5.3 will be officially released in early February.
More information on MUFFIN and screenshots of these four different UI concepts can be found via this Document Foundation announcement as well as more details via TDF design blog.