There are many ways that evolution occurs. "Genetic Drift" is one of those mechanisms.
"Genetic Drift" predicts that
chance causes individuals to leave behind a different number of children (and genes). The genes of the offspring will likely be the genes of those parents who had more children.
Genetic drift affects all populations because there is no way for a population to avoid CHANCE.
For example: a population has 3 green and 6 brown beetles. One day, you accidentally step on a group of those beetles.
Now your population only has 1 green beetle and 6 brown beetles. The random boot stepping on 2 of the beetles has changed the "genes" available in the population.
The offspring in this population will more likely be brown than they will be green.
Genetic drift doesn't make a living thing adapt to its environment, but it is one of the ways in which the "genes" in a population is changed. It can reduce the variety that is present in the population. This changes what traits are eventually available for the living thing to evolve with.