Sometimes abbreviated as MR, mixed reality is a term that describes the blending of a real, physical environment with a digital one. Those familiar with AR (augmented reality) and VR (virtual reality) can think of MR as a mix of the two technologies.
Technologies like Windows Mixed Reality examine a real-world environment, such as an office, and all detected surfaces (e.g., tables and walls) in that environment. Once the environment is mapped, it can be changed using MR, and virtual items can interact with those surfaces. For example, you could use MR to be in an office overlooking the ocean instead of an office with no windows.