The key concept that we need to learn from food chains, food webs and food pyramids is the transfer of energy. Furthermore, only plants can change this energy into a form that can be used by animals.
All plants rely either directly or indirectly on plants for their energy.
Only plants have the ability to convert sunlight into food that they and animals can use. Plants, then, are at the first feeding level. Plant eaters or herbivores feed upon the plants. This is the second feeding level. The carnivores that prey upon the herbivores are at the third feeding level and so on.