This is a rule that states only ten percent of energy available at one trophic level is actually passed

only 10% of available energy is actually passed from one trophic level to another ,the rest is lost in form of heat.

Explanation:

as the energy is transferred from one trophic level to another i.e from producer to primary consumer and so on ,between 80 to 90% of orignal energy is lost as heat as a byproduct of respiration .however a continuous flux of energy from the sun prevents the ecosystem from running down . For example : if the producers have 10,000kcal of available energy then primary consumers would have 1000 kcal of energy and secondary consumers would have 100 kcal of available energy Hence shows that only 10% energy is left at each trophic level.