Frame rate
Frame rate is measured in frames per second (FPS). It is a measure of how many frames your graphics card (GPU) can send out to the display every second.
In order for virtual reality or mixed reality applications to perform well and not cause discomfort (Motion sickness and Cyber Sickness), especially if scenes contain movement, it is recommended to try to achieve as close to 90 frames per second as possible. For static scenes, 60 frames per second can be sufficient for shorter durations (a few minutes).
To reach these kinds of frame rates consistently, a powerful PC and typically the latest generation of VR supported graphics cards are required. It is possible for PC hardware requirements for VR and XR headsets can be significantly reduced by leveraging cloud-based computing solutions.
The term refresh rate is sometimes used interchangeably with frame rate, but the two are not the same thing, even if they are closely related. Where frame rate tells you how many frames your computer can send to display, refresh rate dictates how often your display itself is capable of renewing the displayed image.
No matter how powerful of a graphics card you have, it will not matter if the headset itself does not have a high enough refresh rate to display those frames. Varjo XR-4 Series headset users have the option of switching between 90Hz and 75Hz passthrough frame rates for varied use cases if using at least Varjo Base version 4.3.
Additional resources