A gene pool is just the set of genetic information that collectively defines a species.
Individuals vary, and not surprisingly, their genetic information does too. Mostly the variation is fairly unimportant - is blond hair really better than red hair?
The genetic information turns out to be in little parcels which we call genes. Some variants (alleles) of genes are pretty common, some are rare. If you check all the individuals, and count up how many have which alleles, then you have measured the frequency of the alleles in the gene pool. If a frequency changes, then we say that the gene pool has changed.