What do you understand by conservation of mechanical energy?

The sum of kinetic energy and potential energy of a system is called mechanical energy. For a given system, the sum of kinetic energy and potential energy remains constant. When kinetic energy increases then potential energy decreases and vice versa is also true. This explains the conservation of mechanical energy of a given system. This can be understood by example of a stone kept at a height. The potential energy of stone is maximum and kinetic energy is zero at that height. When the stone is in free fall, its kinetic energy increases and potential energy decreases. When the stone hits the ground, its kinetic energy becomes maximum and potential energy becomes zero. Throughout this, the mechanical energy remains constant.