Whenever anything happens, energy is conserved. What does this mean about energy before and afterwards.

It means that energy is never lost, only transformed into other forms of energy. If a car accelerates, chemical energy stored in the fuel is transformed into heat and movement. If you brake your car, the friction that is created transforms movement into heat. All the energy that was stored in your fuel will still be around if your emptied your tank. It just is not as easily accessable anymore, because it has been stored as heat, exchanged via your radiator, your brakes, your tires and everything has some sort of friction and also movement in the air you pushed around. But every single bit of it is still there.