When herbivores eat green plants they are taking energy into their bodies. The herbivore will use this energy for movement & other body activities, eg: reproduction.
Parts of the plant which was eaten cannot be digested by the herbivore; the energy in these parts passes out as waste. Some of the energy, is used for growth & remains as organic matter in the herbivore's body. It is this energy which can be eaten by the secondary consumer.
Stored energy is eaten by the secondary consumer.
Only a very small amount of the producer's original energy is stored by the secondary consumer. This energy is taken into the body of the tertiary consumer. So the amount of energy available for the tertiary consumer is only very little of the energy which the primary consumer gained from the plant.