A food chain is a linear representation (a model) of how energy is dispersed through the trophic levels of an ecosystem. All food chains follow the same linear pathway
sun --> producer --> primary consumer --> secondary consumer --> tertiary consumer
Below, you will find an explanation of the above terms.
All energy on Earth ultimately comes from the sun.
A producer is an organism that can produce its own food, such as algae or plants. They use the sun's energy to make the food via photosynthesis. The sunlight energy is captured during photosynthesis. Photosynthesis uses the sun's energy to convert carbon dioxide and water into glucose (a sugar) and oxygen gas.
Primary consumers are things that eat the producers (plants). They are sometimes referred to as herbivores, things that eat only plants. Examples would be rabbits and mice.
Secondary consumers eat the primary consumers. Examples would be small predators, such as fox or cats.
Tertiary consumers are the major predators of the ecosystem that eat the secondary consumers. Examples would be coyotes, wolves, and bears.