From the announcement, MUFFIN stands for: My User Friendly Flexible INterface. They break it down this way (quoted):
My: LibreOffice users want a “personal” UI, with different options capable of adapting to the user’s personal habits, and not a single UI without options.I think this is an interesting and welcome direction for LibreOffice. I don't think the current user interface is bad, but I think the proposed changes are a positive step forward. The new MUFFIN interface is flexible and supports users they way they want to use LibreOffice. I think it will appeal to current and new users, and "lower the bar" for users to come to LibreOffice from Microsoft Office.
User Friendly: of course, any UI should be as user friendly as possible, but LibreOffice users have clearly asked for a “modular” UI, where they can set their own level of user friendliness, and not a single UI without options.
Flexible: the increasing number of LibreOffice users deploying the software on different hardware platforms (for instance, a desktop and a laptop), each one with different characteristics and screen size and resolution, have asked for a UI that can be tweaked to leverage the screen real estate, and not a single UI without options.
INterface: The MUFFIN concept is the combination of different UI elements, which are going to be available starting from LibreOffice 5.3 either as a standard or experimental feature.
To give an example, let me examine two options of the MUFFIN interface: Standard and Notebook.
If you're a current user of LibreOffice and are happy with things they way they are, I think the Standard toolbar is for you. It looks pretty much like the menu interface you use today.
Notice the placement and arrangement of menus is very recognizable. I imagine people who have used LibreOffice for years and don't want to change will prefer to stick to the Standard interface.
Then there's the Notebook toolbar. This is very similar in concept to the Microsoft Office Ribbon. People who come from an Office background and are used to how Ribbon behaves - and how it changes based on what you are working on - should like the Ribbon.
Here's a mock-up of the Notebook toolbar, from the Design blog, showing the Notebook toolbar under different circumstances:
I like how most of the Notebook toolbar remains the same (File, Clipboard, and Formatting) and options at the end change based on the context of the document (Tools, Table, Image, and Chart). And note how when manipulating an image, the formatting options are grayed out.
Overall, I'm very pleased and think the new design looks great!