HoloLens is a headset that can project pictures and videos on to the physical world. The headset has built-in sensors and cameras that map the world around the user, and choose where the virtual elements should be placed accordingly.
Typically, these elements are interactive, which means that it’s possible to control them with voice or gestures. For examples, you can zoom in on a plant to see what it looks like from the inside, even on a molecular level.
The benefit of HoloLens is that you can move around freely while wearing it. The only thing you need is an internet connection.