Most mobile devices — such as smartphones — are always searching for wifi networks they have been connected to in the past. Your phone is basically yelling every name of every network it has ever been connected to — at home, at the office or at that hotel with the dodgy connection — to see if it will get a response from the router.
These messages contain enough unique information to be used to fingerprint and track individuals, something that is already being done by many different parties, and for various reasons. Shops for instance, use this data to track how many people walk by, how many actually come into the store, and how much time you’ve spent in the candy aisle before making your choice.
Linger is a small, portable device that allows you to create and blend into a virtual crowd by storing the specific wifi signals from everyone that comes near you, and rebroadcasting their signals infinitely when they leave, making it seem as if they are still there. As you pass people in the street and their signals are stored in the database, a small display on the device automatically updates to show the number of unique individuals in your group.
Physically they may have left, but their virtual presence will stay with you forever.