It's all about Office and Office Business Application (OBA). MS has leveraged Office and exposed the ability for Office to be a first-class application hosting environment. Through Visual Studio Tools for Office (VSTO), Microsoft Office has finally fulfilled the promises of VBA. It is exciting and impressive the amount of Office UI extensibility available for app developers to access. Things like the ability to add application custom content panes (similar to a forms in VS) to office apps. They have also exposed hooks into the UI: Ribbon Bar, Context Menu, … that allow for customization so the resulting Office “hosted” business application are not dumped in, but tightly integrated. The end result looks as much or as little like Office as the app developer desires.
And the Office "Ribbon Bar" is now referred to as "FluentUI." One way to tell if Microsoft is energized about a technology is if the name of that technology changes multiple times. Looks like FluentUI fits that bill.