Genetic drift is a part of microevolution.
Microevolution is the change in the frequencies of the alleles in a population that occurs over a very short period of time in contrast to macroevolution, that occurs over a large period of time.
There are four different processes that contribute to microevolution :
1. Mutation
2. Selection
3. genetic drift
4. gene flow
Genetic drift leads to the change if frequencies to random sampling based on a chance event. The effect of genetic drift occurs suddenly and, may be beneficial, detrimental or neutral.
For e.g in a population you have red and green beetles but, suddenly one day an elephant comes into the field and, crushes most of the red beetles so that now the allele frequency changes and, we have a greater number of green beetles.
This example shows how the allele frequency changed suddenly over a short period due to random sampling and, thus this shows how genetic drift causes microevolution.