The Photo Input node is used to load media shot by digital still cameras, and define the parameters of the clip and camera that are passed down the tracking tree for photogrammetry.
Further information about the individual components of the Photo Input node are available in the Media Management section:
Source: The source media information. This is also where media can be loaded from disk.
Undistort: The Undistort panel where lens distortion correction parameters can be defined.
Position: The GPS metadata viewer panel.
Active Metadata: The active metadata window.
Camera Body: The camera body panel.
Camera Lens: The camera lens panel.