In the context of aerial photography, fiducial marks are small registration marks located along the outside of an aerial photograph. There are typically four or eight numbered marks, which look like this (note: this is number 1):
So what are they for? During the camera calibration process, the positions of the fiducial marks are measured precisely. The principal point of the image can be derived from the intersection of the fiducial marks. See here for an interesting paper on the development of camera calibration methods. The results of the camera calibration are usually stored and reported in a document (typically a USGS camera calibration report in the USA), and some organizations include their camera calibration reports on their web-sites: here is an example.
Fidicial marks are also important in the early stages of the photogrammetric processing, when the system establishes the relationship between "film" coordinate space and "pixel" coordinate space (solving for interior orientation). This process involves either physically or automatically measuring the fiducial marks.
Finally, it is important to note that fidicial marks are only used in film cameras. You'll only see them on scanned aerial photography. Digital cameras use different camera calibration techniques, and the USGS has a research lab on digital camera calibration research.