Plain English Meaning
How fast the airplane is going and how high it is together make up its energy. The pilot manages that energy by using the elevator and the throttle together. The elevator shifts energy between speed and height, and the throttle adds or removes energy overall.
Full Definition
A foundational rule of energy management stating that an airplane's total energy state is determined by the combined use of pitch (which trades altitude for airspeed or vice versa) and power (which adds or removes total energy through thrust). Neither control alone governs the airplane's energy; they must be coordinated. Pitch redistributes existing energy between altitude (potential energy) and airspeed (kinetic energy), while power changes the total amount of energy .