The graph shows the final velocities of two objects after a direct impact.
After the impact Kinetic Energy can not increase and this means that the final velocities must be within the "Energy Ellipse".
In addition, momentum must be conserved and therefore the final velocities must lie on the Conservation of Momentum line.

You can change the masses of the two objects and their initial velocities. You can also change the coefficient of restitution e. If e is equal to 1, the masses are very bouncy and kinetic energy will be conserved after the impact. If , the objects will coalesce after impact.
Can you write down the Conservation of Energy and Conservation of Momentum equations? Can you use these to work out the possible final velocities?