Mechanical energy is the sum of macroscopic translational and rotational kinetic and potential energies. Mechanical energy is the energy that is possessed by an object due to its motion or due to its position; can be either kinetic energy (energy of motion) or potential energy (stored energy of position).
Objects have mechanical energy if they are in motion and/or if they are at some position relative to a zero potential energy position.
The principle of the conservation of mechanical energy
The principle of the conservation of mechanical energy states that the total mechanical energy of an isolated system remains constant in time, as long as the system is free of friction and other non-conservative forces. In other words a conservative force as a force which does not change the total mechanical energy, which is true, but might shed much light on what it means.
In any real situation, frictional forces and other non-conservative forces are present, but in many cases, their effects on the system are so small that the principle of conservation of mechanical energy can be used as a fair approximation. Though energy cannot be created or destroyed in an isolated system, it can be converted to another form of energy.