In food chains we find animals that feed on plants: they are called animals herbivores.
Other animals eat herbivores: they are carnivores. And there is still carnivores that eat other carnivores and animals that eat both plants and other animals, being called omnivores. All these organisms that feed on other beings are called consumers.
For simplicity we call the first consumer in the chain, that is, herbivorous animals, primary consumers or first order consumers. The next animals are classified as secondary consumers. The following are tertiary consumers, quaternaries and so on. There may be fifth-order consumers or more, but the chains do not go much further.