Biogeography is a branch of biology that focuses its efforts on studying the geographic distribution of living organisms. The study is focused on habitation patterns as well as the various factors that could have been responsible for the current distribution of life we currently have on Earth. Biogeography also provides evidence for evolution through natural selection.
Earth is a dynamic planet. It is changing and it has changed throughout history. Continents move, new islands emerge, natural disasters happen, and so on. This dynamic environment forces species to adapt. They adapt to new environments, and they find new niches to be successful in. As the organism becomes better and better adapted to a particular environment and niche, the species may begin to change and differentiate itself from previous iterations or other similar organisms.
Biogeography helps track those kinds of changes based on geographical location. Probably one of the most famous examples that intertwines biogeography and evolution is Darwin's Galapagos finches. At some point in Earth's history, no birds existed on the islands. Finches were an absent species. Sometime after that, the first finch arrived on one of the islands. The islands are not identical to each other, so the birds adapted to the various niches and evolved to become the different species of finches that we see on the Galapagos islands.