R/K selection is a theory about how different species employ different reproductive strategys, in order to maximise their evolutionary fitness. R-selected species have many offspring, with low parental investment, which means survival rates for the offspring are low. K-selected species have fewer offspring, with high parental investment, which means survival rates for the offspring are high.

It must be kept in mind that R/K selection is not a binary yes/no phenomenon: It exists on a spectrum, with lower animals employing extreme forms of R-selection, and higher animals employing extreme forms of K-selection. Compare insects that have thousands of offspring, to primates that can only have one offspring at a time.

Different degrees of R/K selection can also be found within different populations of the same species. This becomes clear when you look at the reproductive strategys of blacks vs whites. Blacks have more children with lower parental investment, whereas whites have fewer children with higher parental investment. This is why the theory is so controversial.