A pyramid of energy represents the movement of energy through an ecosystem. It shows the amount of energy at each trophic (feeding) level.
It is basically like a food chain in the sense that it begins with a producer or green plant, followed by an herbivore (primary consumer) which is followed by a carnivore (secondary consumer) continuing until all later consumers are accounted for.
Because the greatest amount of energy is captured by producers via the process of photosynthesis, the plants will be at the bottom of the pyramid of energy which is the broadest and largest part. Energy is lost at each level and there is only 10% of the energy on each succeeding level, as compared to the energy in the level below it. Once food has been consumed, an organism has to use much of it to carry out life functions such as respiration, growth and repair, but most of the energy in food is lost to the environment as heat.
Therefore, if 100% of energy is at the producer level, there would only be 10% at the primary consumer level, 1% at the secondary consumer level, 0.1% at the tertiary consumer level, etc.
This pyramid of energy can also be thought of as a pyramid of numbers. If plants were at the bottom of the pyramid, rabbits could be next, followed by snakes and then hawks at the top of the pyramid. The amount of available energy decreases as we move up the pyramid as does the number of organisms.
I have included a link with a diagram of an energy pyramid.