This is the part 2 of Experiment 18. It's a "reaction tester". By using 555 timers, there's a delay start to a "counter". First one presses a switch and a few seconds later, the LED goes off and it starts counter. The game is that, whenever the LED goes off, the user presses the button as quickly as he/she can. The numeric display shows how long in milliseconds (not necessarily very precise, though) to react.