Frames per second, also sometimes referred to as Frame Rate or the acronym fps. This measures how often images are shown in the headset in sequence.
A faster frame rate improves the VR experience for the user.
The frames per second are different from the refresh rate which describes the hardware capability. FPS is specifically related to the performance of the CPU and GPU which is driving the graphics on the system.
A system can have a lower Frame rate even though the hardware refresh rate may be capable of 90 Hz or more.
A Low FPS can cause latency which can invoke VR sickness so should be avoided.
Higher frame rates can be achieved by having a trade-off with better graphics i.e. running game graphics at lower settings.