Random modification of existing genetic variation.
This is modification that occurs independent of both ongoing mutation and ongoing migration, though they, like, genetic drift, can modify existing genetic variation as well. Natural selection too can modify existing genetic variation (i.e., change allele frequencies), but does so in a directed (deterministic), i.e., non-random manner, versus the stocastic variation in allele frequencies that results from genetic drift.
Genetic drift in particular can, though does not necessarily, have the effect of causing detrimental alleles to increase in frequency as well as causing beneficial alleles to decrease in frequency. Natural selection, by contrast, has the effect of causing detrimental alleles to decrease in frequency while causing beneficial alleles to increase in frequency.
In terms of genetic drift, see: impact of population size, impact on allele frequency, impact on genetic variation, and impact on allele fixation.