"Although an individual organism is the one that has to cope with a changed environment ,it is at the population level that natural selection operates to evolve the desired traits". What does this statement mean?

This statement means that variations occur at the individual level which further lead to natural selection in the population. Only the variations occuring in individuals making them suitable to cope up with the changed environment will survive. It is based on "survival of the fittest". 

Further, it may sometimes lead to death of all the individuals showing a certain trait and survival of the ones with a variable trait. This leads to evolution of the species.