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Genetic drift is a change in gene frequency within a population due to chance instead of natural selection. If the population is small, then there can be considerable genetic drift, which can change the gene frequency quite drastically. Think of the genetic variation of a population as a bowl of jelly beans with all sorts of colors, representing healthy genetic variation.
The random sampling style of genetic drift shrinks the gene pool and therefore alters the frequency the alleles are found in the population. Some alleles are completely lost within a generation due to genetic drift. This random change in the gene pool can affect the speed of evolution of a species. Instead of taking several generations to see a change in allele frequency, genetic drift can cause the same affect within a single generation or two.