The credencial is the pilgrims' 'passport' on the Way, a 14-page, accordion-pleated document, obtainable in advance or during the pilgrimage (see list here). Besides some of the pilgrims' personal details, it provides space for collecting stamps.
Pilgrims also have to keep to the rules listed in the credencial.
Collecting the stamps of albergues and refugios in one's credencial is necessary to prove one has accomplished the pilgrimage and so can obtain the compostela. You might also meet some people who are voluntary helpers of pilgrims on the Way. Stop and enjoy their stories and their help, and do not forget to ask for a stamp!