This animation shows the random motion of molecules within a gas. The average speed of the molecules is constant and this therefore represents a gas at constant temperature.
No energy is lost when the individual molecules collide with the walls of the container. The motion is completely random - pressing f5 to re-run the animation will give a totally new set of molecular motions.
One defect of this representation is that it does not show any intermolecular collisions. In a 'real' gas the molecules would be continually undergoing elastic collisions with each other.