Its basically the look of windows.
You can see here:
Fluent Design System
They have some very short examples of what is all about. Basically its more animation to things, 3d parallax and depth, light as a highlight instead of just selection boxes and real life type materials like frosted glass. The last one, scale is probably the most hard to grasp. Essentially they want UI functions to cross from no visual, to 2d, up to 3d. A sort of blending. Probably the most ambitious element.
They have direct xamarin api calls for these functions, so these things will be easy for programmers to add to UWP apps. They demo'd using one of these calls to create a picture that "exploded" out, when you changed screen.
Material design is androids look they came out with a few years ago. The two are not really that similar at all. Material design focuses on drop shadows for example, whereas fluent design focuses on parallax and perspective. They both take inspiration from real life, and they are both supposed to make the OS look better, and be more engaging.