The flow of energy generally is as follows:

Sun → Producers → Herbivores → Carnivores

Since, flow is progressively from one trophic level to another and does not revert back, it is said to be unidirectional, i.e. from sun to plants, plants to animals, animals to other animals and organic remains to decomposers. Thus, the available energy decreases in the higher trophic levels making impossible for energy flow in reverse direction.