Invariant of Motion: Understand in Plain English

[mazed]

Can anyone please describe me the following theory in plain english. Your help will be much appreciated.

"In certain types of systems there is an invariant of motion which is equal to the sum of the kinetic energy and the potential energy of all the particles in the system. This quantity is the mechanical energy of a dynamic system, and in a conservation system it remains constant."

 

[Doc Al]

It's saying that in some systems mechanical energy is conserved.