What is a gene pool?
The set of genes in an interbreeding population.
We can refer to the gene pool of an entire species or we can discuss the gene pool of a certain population. For example, you could study the gene pool of all African elephants found on the planet. You could also study the gene pool of the populations of orcas ranging around the Pacific Northwest of the United States.
In the image below, you can see that the gene pool of generation 1 is different from generation 3 in regards to the R allele and the r allele: