Biologists sometimes say that “natural selection depends on the specific environment where a species lives.” what does this statement mean?

Natural selection is one of the forces of evolution and the enviroment where the species lives is the selection agent. For example, suppose a mosquitoes population in a environment free from insecticides, in this environment there's a high frequency of non-resistant mosquitoes because the environment is not  exerting any pressure on the resistence trait. But when the environment changes and we use a insecticide upon the mosquitoes population, the populations changes because the non-resistant ones die but those resistant survive and beggin to reproduce more effectively. That's natural selection, differences in survival and reproduction between individuals with different phenotypes (traits) and this differences depend of environmental changes.

Traits can be helpful or harmful. If populations of a species are in different environments, some traits that are helpful in one environment might be harmful in another environment