Fiducial markers are visible objects placed in an imaging system (camera) as reference points or measuring units. In VR and AR, fiducial markers are used for positional tracking of HMDs and Input Devices. Fiducial markers can be utilized in both inside-out tracking and outside-in tracking.
In inside-out tracking, markers such as QR codes can be placed in the environment to serve as reference points.
In outside-in tracking, markers such as light-emitting diodes can be placed onto the object so it can be tracked by the external camera(s). The camera or multiple cameras would follow the movement and position of the marker to figure out its position and orientation in 3D space. Different markers can be placed on different objects so they can be tracked independently.
Tracking systems that do not use fiducial markers are called Markerless tracking.