the question is asking about the energies. when the two cars are moving towards each other they are accelerating meaning they have kinetic energy. when they collide; on rebounding, the car regains a smaller amount of kinetic energy – this time travelling backwards. So the overall process is that kinetic energy becomes heat and spring potential energy, which results in heat, smaller kinetic energy and, hopefully, surviving occupants. so the speed of the car before and after will show the energy change
