Target Balls were orange-sized, spherical items which were thrown at random in the Battle Rooms during practice and then shot at with the Flash Pistol to improve accuracy. They could be retrieved using a Ballcaller.
The target balls would light up white, then turn red if shot. Eventually, they would turn back to white. The goal was presumably to make them all red at once, but even Petra Arkanian couldn't do this while teaching Ender Wiggin to shoot the Flash Pistol.