According to Facebook, they propose that in order to achieve this level of thinness, it will require a mixture of holographic optics and polarization-based optical folding. The end result are displays that only measure less than .35-inches thick, which allows them to be housed in a much more smaller compact frame.
However, before you get too excited at the prospect of a pair of light-weight VR glasses, it is a proof-of-concept meaning that while it works, there is still a lot of work that needs to be done. For example, Facebook notes that the glasses can only output in monochrome right now, but in theory, it should be capable of providing a wider color range and also better solutions that can potentially eliminate any visible pixels.
It is still extremely promising though and while VR isn’t quite gaining the same amount of traction compared to AR, hopefully the smaller footprint could persuade more people to adopt VR in the future.