With "credentials" pilgrims are eligible to stay at albergues along the Camino trails in Spain. Albergues include beds, showers, food, and hospitality for very low costs!
Credentials can be obtained at pilgrims' offices, post offices, sites along the Camino, Camino associations, and American Pilgrims on the Camino.
I obtained my credential in St-Jean-Pied-de-Port, France at the Pilgrim's Office. I was required to show the credentials to every albergue and receive a stamp from them. This proved I walked more than 100 kilometers once I arrived to Santiago and to Finisterre which made me eligible to receive a compostella! Walking pilgrims also earned beds in the albergues before biking pilgrims.