A species is "native", or indigenous to a given region or ecosystem, if its presence in that region is the result of only local natural evolution. In other words, it was not introduced into the area by humans (the "unnatural" species).

Native plant species (the focus of this page) are the ecological basis upon which life depends, including birds and people. Without them and the insects that co-evolved with them, local birds cannot survive. For example, research by the entomologist Doug Tallamy has shown that native oak trees support over 500 species of caterpillars whereas ginkgos, a commonly planted landscape tree from Asia, host only 5 species of caterpillars. When it takes over 6,000 caterpillars to raise one brood of chickadees, that is a significant difference.

Source: audubon.org