WHAT DO MEAN BY ENERGY ?

ENERGY is defined as the CAPACITY of a body to do WORK, by virtue of the position or condition of the body.

Now suppose there is a 150-pound plate of steel on your bench, resting on four blocks which are 2 inches tall (so the space between the bottom of the plate and the bench is 2 inches). You install the compressed spring into that space and locate it at exactly the CG of the plate, and release the spring compressor.

The spring will lift the steel plate 3/4 of an inch, so the spring has done WORK on the plate, thereby releasing some of the ENERGY stored in the spring.

There are many different forms of energy. There are a few which are of particular interest with respect to powerplants: kinetic energy (the energy contained in a body by virtue of its velocity), potential energy (the energy contained in a body by virtue of its position), chemical energy (energy which can be released by a chemical reaction, such as combustion), and heat energy (energy which can be used to make machines operate).