Genetic variation generally refers to the differences in genes between individual members of a population, or the frequency in which the various gene types are expressed. Genetic variation is incredibly important for the survival and adaptation of a species, as it helps in terms of natural selection and evolution.
In populations with very stable genetics, where there are few differences in genes between individuals, genetic variation can occur in one of three ways. The first way is through simple mutation, a change of a certain gene or genes that can then be passed on to the offspring. The second way is through individuals from other populations with different genetics entering the original population. The final way is through breeding within a population that results in different genes, usually recessive, being expressed.
If a population has a high genetic variation, it is much more able to adapt to changes in the environment, while those populations with a uniform genetic makeup may be susceptible to mass die-off or extinction if their genes aren't conducive to the new environment. This is the basics of natural selection, as those with the most desirable or healthy traits live, while those with other traits may perish. With low genetic variation, natural selection becomes much more difficult, often leading to extinction.