Geographical isolations happen when two populations get separated by a geographical feature like lakes, seas, rivers, land, islands, mountains and etc.
Due to the geographic features, the populations separated evolve into different characteristics. Because of that, the two separated populations become two different species, and when that happens, it's too late for them to be able to interbreed.
So they don't usually breed, but if they do, it won't work. Due to how different the species are, it is too late to work and the offspring will be unable to live.