
Your PC stores environment information so the tracking system can recall environment-specific data like the room boundaries physical location.

The pose information is used by both applications to render a scene and by the system to correct this rendering for any mis-prediction in time and position. The system will triangulate its position based on the observed features, which then supplements the information by fusing high rate IMU data to produce a continuous pose estimation for the HMD in your environment. More details: The tracking system uses two low-resolutions black and white cameras to identify features in your environment in visible light. The cameras then fuse the information with IMU data to determine a precise position of the device in your environment. Quick answer: the tracking system uses two visible-light low-resolution cameras to observe features in your environment.

In this article How does inside-out tracking work?
