The ability to do work is called energy. Most activities in modern life require energy.
Following is a detailed discussion of the Law of Conservation of Energy.
Law of Conservation of Energy
Energy is indestructible, energy cannot be recreated or destroyed. Only energy can be converted from one form to another. The total amount of energy in the world is constant.
Example
When an object is dropped from a height without stopping, the value of its mechanical energy (kinetic energy + potential energy) remains constant at any position. All the energy of the object in the first position is potential energy. As the object falls downwards its potential energy decreases and kinetic energy increases.
But the sum of the potential and kinetic energy of the object at any given position is equal to the initial potential energy. Just before it hits the ground, all its energy is kinetic energy, but the value of this kinetic energy is also equal to the initial potential energy. So, the total energy is always the same.