Complex sentence: Over longer periods of time, genetic diversity spreads the population out over a larger area of genotype space, giving the population mutational access to a greater number of distinct heritable phenotypes that are reached from different points of the genetic neutral network.
Split into simpler sentences: Over longer periods of time, genetic drift causes the population to spread out over a larger neutral network in genotype space. This genetic diversity gives the population mutational access to a greater number of distinct heritable phenotypes that can be reached from different points of the neutral network.