The principle of conservation of energy states that energy is neither created nor destroyed, it is just transformed from some types of energy into others. The following visual will help you to understand what is meant by the principle of conservation of energy when a body falls.

An electric motor receives electric energy in the form of an electric current and produces a movement (mechanical energy).

The theorem of conservation of mechanical energy states that the work carried out on a body is invested exactly in increasing some type of energy. The visual simulates a moving train.