Also known variously as crédential, crédentiale and crédencial, this is a sort of religious passport issued by the church, entitling the holder to stay in pilgrim gîtes along the way, and to receive a certificate of completion on arriving at Compostela.
At each stage of the journey, the pilgrim gets a stamp (tampon) as proof of his having spent the night there. In French gîtes, most guardians are easy-going about the créanciale and will happily stamp a guide book or a piece of paper, but in Spain things are much more strict.
The origin of the créanciale goes back to mediaeval times, when it served as a letter of introduction and sometimes as a true passport to get into Spain in times of war. The custom was revived in the 1950s after several pilgrims were charged with vagrancy as they made their way through the countryside, and needed a document to prove that they were not a threat to the local citizens.