It is easier to understand the gene pool if we look at an example, so lets use dogs.
All dogs can mate with each other. They have the same set of chromosomes with the same set of genes. So why do they look and act so different?
Dog breeds are different because of mutations in the DNA sequence. The different versions provide slightly different instructions.
The gene pool for dogs is simply all of the versions of all of the genes that dogs have.
We say a gene pool is large if there are many different versions of genes. Some versions may increase an organisms chance of survival. Organisms with that version live and reproduce. This is the basis for evolution.
The size of the gene pool can change. The gene pool gets larger when organisms have mutations that are passed to the next generation. The gene pool gets smaller when all of the organisms with a particular allele die before having offspring.When the population of an endangered species goes down. It has a shrinking gene pool. This makes it more difficult for the plant or animal to adapt to changes in the environment.