Eye-tracking is the process of measuring the position and movement of the user’s eye within the headset to determine exactly where the user is looking.
This is something the current generation of consumer headsets, including the HP Reverb G2 do not currently support. Incidentally, HP has released an enhanced version of the Reverb G2 know as the Omnicept Edition which does include it and is targeted at enterprise level solutions.
Eye Tracking is a critical part of implementing the technology known as foveated rendering.
Undoubtedly this is something that will be a common feature of the next generation of HMD displays and will provide great enhancements to the visuals and other VR functions.