Fiducial marks are special marks that are used by machine vision systems to automatically align themselves to a PCB.

Some manufacturers require at least two ficucials to be located on your PCB design. Fiducials are normally 1mm diameter copper circles in the center of a 10mm keepout zone (this includes soldermask). Put the fiducials in opposite corners and as far away from each other as possible for the machine alignment to be most effective.