Let's look at how energy is transferred through a food chain. Energy from the sun is harnessed by producers, such as this plant, through photosynthesis. Some of the energy is used by the plant for cellular respiration, or is lost as either heat or waste.
The rest of the energy is stored in the body of the plant. If the plant is eaten... Only the energy stored within it is passed on to the consumer, in this case a caterpillar. Of that energy from the plant, some will be used by the caterpillar for cellular respiration, or lost as either heat or waste, while the rest of the energy is stored in the body. If the caterpillar is eaten by another animal, it is the stored energy in the caterpillar that is passed on to the next step of the food chain.
In this way, energy is transferred up the food chain in one direction, but only the stored energy is available as food for the next level in the food chain.