One of the characteristics of life is that it reproduces itself. Since nothing lives forever, there is always a turnover of generations. In a stable population, the birth rate equals the death rate, and the total numbers do not change. If the death rate is higher than the birth rate, the population will get smaller and smaller until finally it becomes extinct. If the birth rate is higher than the death rate, the population will grow larger and larger until it finally reaches some limit that causes the births to decline or the deaths to increase.
Populations have the potential to grow very rapidly, in what is called a geometric progression.
The speed of population growth depends on the number of offspring per generation and the length of time between generations. If the above figures applied to a couple of human beings with 25 years between generations, it would take almost 400 years to reach a hundred thousand. If the same figures were for cats breeding at two years of age, the pair of cats would reach 100,000 in a little over 30 years. Obviously something must happen to limit those populations before they get so large. On the other hand, a green turtle that takes 40 years before being old enough to lay a hundred eggs of which only 2 survive to grow up will just succeed in replacing itself if it is lucky, and just a few adult turtles killed would mean the collapse and disappearance of the population.