Every organism needs to obtain energy in order to live. For example, plants get energy from soil and the sun, some plants eat animals, some animals eat plants, and some animals eat other animals.
The food chain cycle is a series of trophic levels that begins with autotrophs (auto means self and troph means nutrition in Greek). These autotrophs are primary producers that gain their main source of energy from sunlight or a boiling-hot deep sea vent. If they gather energy from sunlight they would be called phototrophic autotrophs, and if they acquire their sustenance from a sea vent then they would be called chemotrophic autotrophs.
Autotrophs (or primary producers) are consumed by herbivores (or primary consumers), who are eaten by carnivores or omnivores (secondary consumers). Secondary consumers may be eaten by tertiary consumers, who may be eaten by quarternary consumers.
When the apex predator dies, it will be consumed by detrivores (like vultures, worms and crabs) and broken down by decomposers (mostly bacteria and fungi), and the exchange of energy continues.
Here are a couple examples. Keep in mind that a cycle never ends. The fungus and bacteria actually compete at the same level, and sometimes other single-celled organisms like amoeba and protozoa eat bacteria.
• algae --> mosquito larva --> dragonfly larva --> fish --> raccoon --> eagle --> fungus/bacteria
• phytoplankton --> zooplankton --> fish --> seal --> white shark --> orca --> crabs/small fish/bacteria
• grass --> grasshopper --> rat --> snake --> hawk --> fungus/bacteria
• algae --> snail --> crayfish --> fish --> alligator --> crayfish/fish/fungus/bacteria