One of the rituals of the Camino is to get an ink stamp in your peregrino credential book at every albergue you stay at. Folks also get them at cathedrals, restaurants, and other sites along the walk. The purpose is to prove that you’ve indeed walked the camino, so you can get (for €3) your official participant “compostela” when you arrive in Santiago:

I don’t care much about the compostela, but I’m going to cherish the book of stamps.