Imagine you want to get 1 kcal of energy from a cow. How much energy would the cow need to get from plants? Why?
In order to answer this question, you must consider trophic levels. Plants utilize sunlight to fix carbon into carbohydrates that they can use for energy. They do not consume living organisms (typically) and are known as primary producers. The cow in this example consumes plants. Because they consume only producers, they are known as primary consumers. Finally, the human (or other predator) that consumes the cow is the third organism in the chain and is known as a secondary consumer (secondary consumers include primary consumers in their diets).
As a rule of thumb, only 10% of the energy from one level (the level of the prey item) can be used by the consumer; 90% of the energy is lost between each trophic level. If you think of it based on percentages, 100% of the original energy is available at the base of the trophic cascade in plants. When primary consumers eat the plants, they get only 10% of the original energy and when secondary consumers eat the primary consumers, they get only 10% of that (or 1% of the original amount of energy). This explains why trophic cascades (food chains, essentially) are not very long (very little energy remains after a few steps).
If a human gets 1 kcal of energy from eating a cow, that means that the cow actually had 10x that much energy (remember that only 10% is passed on to the next higher trophic level). The cow had 10 kcal of energy. Then we consider the plants/producers that provided energy to the cow. Again, the amount of energy must be 10x greater than at the level of the primary consumer. Thus, the cow must have gained 100 kcal from the plants.