Tapp is a very simple game consisting on a button you share with a friend, which can be turned on and off by any of the players. The other player will instantly see the update. It can (and should) be also used as a communication system with your friends, to let them know that you just got the party or that there were some M&M's left in the shop downstairs.
A project which dates back to June 2017 and has not been maintained since then, likely unable to run in newer versions of iOS.