The mechanical energy of a body is the sum of its kinetic energy with its potential energy.
If we consider the case that there are no losses from heat generation, usually due to friction, with a surface or with air, and since energy cannot be destroyed or created, we can say that the Potential energy turns into kinetic energy and vice versa.
To better understand what mechanical energy is, suppose a body will be abandoned at some point. At this moment it has potential energy. When abandoned, as it falls, height decreases and velocity increases, that is, potential energy decreases and kinetic energy increases. If there are no friction losses, this body will gain kinetic energy in exactly the same amount as it lost in potential energy, so the sum of the two always gives the same result.
For example, in a waterfall, water at the top has basically potential energy. As it falls, its potential energy decreases. In contrast, velocity, and hence kinetic energy, increases. This conservation of potential energy in kinetics, and vice versa, follows the principle of conservation of mechanical energy.
In the absence of frictional forces, mechanical energy is conserved.
Now let's look at the following situation: Two athletes lift weights at the gym. They lift the same weight at the same height. However, to do this survey one athlete takes a second, and the other takes two seconds.
Both perform the same work, that is, they spent the same energy, but the first athlete was faster than the second, do you agree?
This relationship of energy to time is called power (P).
The unit of power in the International System is the J / s, which we call Watt (W).