First edition is literally means the first printed version of the book without major editorial works. For a regular reader, it might be hard to understand the importance of first edition. But, for collectors, first edition is the closest bond that they can build with their favorite authors. It is a way to actually posses the work. It is an obsession, but an honest one.
However, does that really have to change the value of the book? Or, who decides a certain kind of book's first edition is so expensive, while another book's first edition doesn't cost very much. When I had my interview with Ahmet Yüksel- Sanat Kitabevi's owner- he said that he doesn't understand how people estimate a price for first editions. However, he sells them anyway.