A gene pool is the complete set of genes found in a population of organisms from the same species.
The size of a gene pool depends on the genetic diversity of organisms in the population. A large gene pool would mean that there is a vast genetic diversity between the organisms of said species, their genetic material vary widely from one another. A small gene pool would indicate a narrow genetic diversity (for instance, if inbreeding for many generations has occured). This can affect the population by reducing the fitness of organisms and their ability to survive. However, there are certain organisms that can still survive and thrive even with a small gene pool, due to genetic drift and new genetic variants.
Graham, Loren (2013). Lonely Ideas: Can Russia Compete?. MIT Press. p. 169. ISBN 978-0-262-01979-8.