mutations - What is the standing genetic variation?

I am reading this review. In the first part, the author introduces Standing Genetic Variation, described as:

STANDING GENETIC VARIATION Allelic variation that is currently segregating within a population; as opposed to alleles that appear by new mutation events

Does it mean that in adaptation the allelic variation is already present (not created by mutations) and in adaptation it prevails over the others?

Thanks

Answer

Standing genetic variation is when there is more than one allele at locus in the population at the time-point in question. When an allele goes to fixation there is no standing genetic variation at the locus until new mutations occur. Loci where alleles are not fixed are described as having standing genetic variation.

"Standing genetic variation: the presence of more than one allele at a locus in a population."

from Barrett and Schluter 2007