Question

What does it mean that energy is neither created nor destroyed

Answer 1

It means the amount of energy in the system stays the same it just transfers or changes form. As a example, think of a light bulb which when given 100 watts of electrical energy will convert 60 watts to energy in the form of light and 40 watts to energy in the form of heat.

This is a hypothetical scenario, idk the real efficiency of a light bulb is low but not exact numbers.
then you can write equations for energy, Luke this one.
100 watts electric = 60 watts light + 49 watts heat

Answer 2

Energy can only be transformed. You cannot destroy energy or create it. For example, the potential energy when you have something sitting on a desk doesn't disappear once it falls off. It gets TRANSFORMED into kinetic energy. This happens constantly. Energy, the ability to do work or cause a change, is always being reused in different forms. Hope this helped!