Question

what is a tertiary consumer

Answer 1

in most chases a carnivore.
plants are the primary producers
herbivores are primary consumers since they eat the primary producers.
The organisms that consume the primary producers (herbivores) are called secondary consumers and the organisms that eat the secondary consumers are the tertiary consumers.

So an example: an insect eats from a flower. the flower is the primary producer and the insect is the primary consumer.
Then the insects gets eaten by a mouse, the mouse is the secondary consumer.
Then fate strikes and the mouse gets eaten by a snake. The snake is the tertiary consumer.