The pilgrims passport, of the Credencial, is what pilgrims bring with them on their walk collecting stamps along the way, proving that hey have walked “all the way”. It is recommended to get two stamps per day if you are walking from within Galicia.
Once you arrive at the pilgrims office in Santiago, before they will give you your compostela, the will check this passport and the stamps to make sure you did not skip stages, and actually walked all the way.
You can get this pilgrims passport at one of the many albergues, or at the pilgrims office in Santiago. Local Confraternities around the world also sends this out for a fee.
Here is a conversation on the Pilgrims passport from the forum.