Its when you get photos taken in the parks by Disney photographers; Disney Photopass photographers. They will then give you a Photopass card and all your photos will be put on it and when you trip is done you can take that card home and use the number on the back to go online and see your photos and pick which ones to buy.
Each photographer you will will ask for your card and swipe it and add your photos taken to that card. The card looks kind of like a credit card.