Evolutionary fitness is measured in terms of reproductive success.
This simply means that the genes of those who have the most offspring tend to beat out, in the long run, the genes of those who leave fewer offspring. This is natural selection at the genic level. To contribute to that gene pool requires reproductive success, and natural selection is generally defined today as change in gene frequencies, or differential reproduction. Genes of course can't be literally, consciously selfish, but it is a useful concept. So it is instructive to think of our genes as driving us into reproductive competition, with our disposable bodies being just temporary containers for these relentless little buggers.