This can mean many things, from restoring a photo to its original state as far as possible, correcting tears and cracks that occur over time, repairing missing corners or joining two or more parts of a photo into one cohesive photo.
Additionally, a colorization process can be applied to the original Black & White or colour image. As time goes by colour prints of all types will fade and the colours will fade at different rates, usually the older colour photo can look a little blue or purple. This can be corrected.
In the case of a Black & white photo, the paper will change over time and look a little yellow or very yellow. This can be corrected, if required.
Also, some of the original Black & White prints were either Cyanotype or Sepia. This was applied to the image parts but not the paper. This can be replicated at the same time as printing on white paper, to bring back the original print intention.