Law of conservation of energy states that the energy can neither be created nor destroyed but can be transformed from one form to another or in other words we can say that the total energy of a system remains unchanged before and after transformation.

For example- A rock on a cliff. The rock has potential energy, when it's pushed off from a height, the potential energy is changed into kinetic energy.

The sum of potential energy and kinetic energy remains constant at every point of the falling of object.

mgh + 1/2 mv²= constant at every point

The sum of potential energy and kinetic energy is the total mechanical energy of the rock falling from a height (cliff).