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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.
Genetic drift is when a population experiences a decrease in variation and population size by chance due to the bottleneck or the founders' effect. For example, a volcanic eruption kills most of the flowers in a nearby area. This bottleneck effect reduces the variety of alleles and traits of the flowers by reducing their number.