Geographical isolation is a result of complete coincidence or accident. It literally means separating individuals based on their geographical location. So the world has emerged in such a way that all the organisms are not located in the same place, rather they are located in different regions of the globe. Not all organisms located in different corners of the globe can interact with each other, owing to their physical distance. Thus, they are prevented from exchanging genetic material and giving rise to progenies. 

An example of this can be the people of Finland, who are secluded to some degree from the rest of the world by water, develop certain diseases due to the lack of genetic material from other races. This is an example of isolation by distance/barrier.