A primary consumer in a food chain is the bottom level of animals that eat plants and grasses for nutrition. Another name for this group is herbivores since they only eat plants and not other animals. The plants use sunlight to produce their own food via photosynthesis. Organisms that produce their own food are called autotrophs. Primary consumers eat the plants for food, and then omnivores and carnivores each the primary consumers for food. The animals at the top of the food chain (including humans) then die and bacteria and fungi (decomposers) then feed off the remains for nutrients and recycle the chemical elements back into nature for reuse.
A primary consumer in a terrestrial setting would be a caterpillar. They feed on plant leaves, and in turn they are eaten by birds and other small animals. A primary consumer in an aquatic setting would be shrimp. Shrimp eat mostly phytoplankton, or plankton that produce their own food via sunlight. Other marine animals like large fish and whales eat the shrimp for food.