Microsoft HoloLens Developer???s Guide

Stationary frame of reference

Spatial or world anchors are references to real-world objects. A coordinate is located in reference to the physical stuff around us. This means we can express the position of a hologram in relation to things that are actually there. So, you would say that a virtual dinosaur is to be placed three meters left of that wall, about 30 centimeters above the floor, and so on. The device will take care of the placement since it knows about these objects. This is what makes HoloLens unique in the world of virtual and augmented devices.

However, you do not always need this kind of behavior. Most of the time you can get away with less precision; especially when you have items that move about, such as following the user ...