Genetic drift is the change in the frequency of alleles in a population over time because of the random sampling of alleles that occurs because of the random processes underlying mate selection and number of offspring.
With each new generation, the random process is repeated, so that allelic frequencies can change (i.e., "drift") substantially over a large number of generations. The figure below, in which alleles in a population are represented by red and blue balls selected from a jar, illustrates how genetic drift can greatly affect small populations. In these populations, the allele frequencies can change much more quickly based off of random chance.
In an extreme case, alleles can become very rare or completely disappear from a population because of genetic drift. One manifestation of this phenomenon is a population bottleneck, in which the number of reproducing individuals is greatly reduced in a short amount of time (by disease, famine, natural disaster, human influence, etc.)
Another cause of large changes in allele frequency changes is called the founder effect, in which a small, isolated population forms, and later generations of the population display alleles held by the founders of the population in amplified percentages.