If a species is K-selected than its population is determined by the number of individuals the environment can support, i.e. if a particular environment can only support 50,000 individuals than that is the limit. In theory, the environment cannot support anymore individuals than that number.K-selected species are typically of higher order, like humans, show parental care, live a long life and have fewer offspring.
The opposite of a K-selected species would be an r-selected species which tend to be of a low order, have many offspring at one time, short lives, no parental care. If I recall, r-selected populations are limited, for the most part, by predators.
congrats on finishing the MCAT's 😉