In ecology, the trophic level is the position that an organism occupies in a food chain - what it eats, and what eats it.
Wildlife biologists look at a natural "economy of energy" that ultimately rests upon solar energy.
When they look at an ecosystem there is almost always some foundation species that directly harvests energy from the sun, for example, grass (however in deep sea hydrothermal vents chemosynthetic archaea form the base of the food chain).
Next are herbivores (primary consumers) that eat the grass, such as the rabbit.
Next are carnivores (secondary consumers) that eat the rabbit, such as a bobcat.
There can be several intermediate links, which means that there can be another layer of predators on top, such as mountain lions, which sometimes eat bobcats.
Since each layer of this system relates to the one below it by absorbing a fraction of the energy it consumed, each one can be understood as resting on the one below - which is called a lower trophic level.
Keep in mind that trophic relationships are rarely this simple.
Very often they are more of a "web" than a "chain." For example, the mountain lion may eat the bobcat, but it also eats rabbits.
The trophic categorization of the mountain lion exists on two levels, possibly more.