A piratebox is a file sharing box. It broadcasts a wifi signal. It will show as an available wifi connection. After someone connects to it, any website they go to will redirect them to the homepage of the box. Similar to how Starbucks, Target, and other companies redirect you to an agreement page when connecting to their wifi. You can put files and information on the home page of the box. There is also a chat feature that can be setup.
There are a couple of ways to set one up. If you have an old Android phone that is rooted, you can use it. Otherwise there are devices you can buy for around $50. One who tried it at a convention said that he saw several people connecting and that attendants were going around trying to figure where the signal was the strongest to try and locate the box.