Let us say you have a population of variant organism in a geographic location that is then divided by something, say a river. Now you have two populations of variant organisms that can not longer interbreed and are still going to have various mutations that not only will be different, but unable to enter each others gene pool. Now your two populations are headed in different genetic directions and this has consequences not only for simple evolution, the change in allele frequency over time in a population of organism, but can also lead to new species. Take whales as an extreme example of geographic isolation. This explanation is quite simplified.