Headball is like Volleyball or Beachball. Bounce the ball with the head over the net. Try to block the ball near the net with your body or jump to smash it into the other side. 
But keep attention! The nasty chinese on the other side knows these tricks too.
Headball provides a one player and a two player mode. For the 2 player mode you can simply connect 2 devices in the same Wifi and play against your friends.