What does it mean to say that an allele is "fixed" in the population?

Alleles are the individual genes that give rise to certain characteristics in an organism.

When an allele is "fixed" to a population, this means that the characteristic will be constant & most likely will not change until the environment changes as well.

Like with arctic hare, the alleles that influence the color of their coats (white in the winter & brown in the spring/summer) will stay because any fur pattern other than the aforementioned characteristics with result in becoming someone else's food with the lack of camouflage.

Hope that helps!