It is all the individuals of a single species. That is, the human population includes only Homo sapiens. The domesticated cat population only includes domesticated cats, and so on. Finally, we need to remember what defines a species. A species is a group of reproductively isolated organisms.
Species can only reproduce with the same species.
A species is an individually reproducing unit, in its simplest sense. At the ecosystem level, species and populations and communities of organisms and their non-living environment organize themselves in ways that maximize or make efficient use of the flow of energy and matter. All energy on our planet comes from either the sun or the internal heat engine of the mantle. The sun provides continuous energy, but the flow of energy is always one way. Once energy is used, it cannot be reused or recovered.