These VR Goggles are part of the “family” of VR headsets that use your Smartphone to generate the VR content. The sensors on board your Smartphone (Gyroscope, Magnometer, Accelerometer) enable the headtracking and positional tracking (if the content allows that is)
The workings of Mobile VR Goggles is basically the same as that of other VR Headsets (like Head Mounted Displays) and here is how that works.
The big difference is that, unlike a VR HMD, pretty much all the computing power and sensory activity needed to experience the Virtual Reality is on your phone.
This is one of the reasons why a Mobile VR headset is so much cheaper than a VR HMD.
The biggest difference in price in Mobile VR Goggles mostly comes from the building quality, material and so on.
The relative low price makes this type of VR Goggles very suitable for people who just want to try VR but don’t want to spend the big bucks already.