Photo Disc

A Photo Disc is a round disc at the center of a PokéStop. They can also be found at Gyms by selecting the Photo Disc button. When a Trainer swipes the disc, a number of items will be given to the Trainer and added to their Items Bag.

Background
Once a trainer has collected items from a PokéStop, it cools down for a short period of time, that is 5 minutes. It turns purple to indicate that it is inactive. Once the cooldown period expires, it will became blue again and items can be collected once again.

To help Trainers explore new places in the real world, PokéStops they haven't previously visited will appear surrounded by rings. Once they spin the Photo Disc, the rings will disappear indicating that they've explored the location.

Trivia

 * Trainers will earn bonus items when visiting Gyms controlled by their team. If they have a Badge for the Gym they're visiting, they will earn additional bonus items as well.