Changes in gene frequency of a population by chance and not by natural selection on mutation.
Small population with certain gene frequency than the rest of population might get isolated or non random mating over long period creates different gene frequency.
Suppose population has frequency of blood groups as O blood group with 90% a with 9.5% and 0.5% B blood group. But somehow a population with 50% B blood group gets isolated then over time am AM 10% a and 20% O blood group.
Amish people migrated from Germany to Pennsylvania in 1720 and remained socially isolated. they have high frequency of rare alleles that cause dwarfism malformed heart and extra finger in hand.
Causes of drift
Natural calamities. Migration. isolation by inbreeding. Behavioral isolation like caste endogamy.
It creates a bottleneck effect where many other genes are limited and only few remain and allowed to thrive. Generally by natural disaster or war
Founder effect when migration.
But if the population starts breathing with outsiders the effect may dissolve.