Answer:
The law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time .
For example, suppose there is a rock at the top of a hill. It has a lot of potential energy because it is placed at a high point. Next, the rock fall downhill, then it speed increases along the fall (at first it speed was zero), increasing at the same time its kinetic energy while potential energy decrease given that the height of its position decrease. Also some potential and kinetic energy is lost because of friction between the rock and the soil.
As you can see, when some kind of energy is lost another kind of energy/energies increase proportionally, and then the total energy is conserved.