The simple answer is spatial coordinate systems. You likely just had a flashback to high school math classes, possibly with a little heavy breathing, which is an appropriate response. However, anyone who works on 3D applications deals with rigid virtual coordinate systems on a constant basis; they are essential for making a 3D program work, be it a game or the new mixed reality fitness simulator.

Coordinate systems are consistent in the computer world. If your coordinates are broken down so that 1 unit is equal to 1 meter in a 3D application, you know that when you move a character to the 40^th unit, you are 40 meters out. This, unfortunately, is not the case in the real world. So, while we do have the geographic coordinate system, which is a combination of longitude, latitude, and elevation, we don't have an accurate way to translate that into a mobile computer in real-time.

Global Positioning System (GPS) is one common option, an option that has been left out of the HoloLens. The problem with GPS--making it completely justifiable to be left out--is that it is not very accurate. Often, GPS can vary by 15 ft-25 ft, which would be terrible for a world-scale device such as the HoloLens. These devices, being designed for the indoors, could not function with that level of imprecision.

So, instead of using a highly inaccurate system, Microsoft went with one whose--while still somewhat inaccurate--imperfections are far smaller in scale: World Anchors.