in small, isolated populations, alleles fluctuate randomly in frequency due to chance events like random mating outcomes and accidents. Alleles can be permanetly lost if the frequency fluctuates to zero. Then the alternative allele is said to be "fixed" E.g. in a Bb x Bb mating that only produces 2 offspring, there is a 1/16 chance that both offspring will be bb. If this is the whole population, B has been lost forever. The larger the population is, the less chance that all offspring in a generation will be bb, so drift is more severe in small populations.