The Great man theory is a theory that explains history by the impact of "Great men", ie: highly influential individuals, either from personal charisma, genius intellects, or great political impact.
This theory is usually contrasted with a theory that talks about events occuring in the fullness of time, or when an overwhelming wave of smaller events cause certain developments to occur.
When this theory is applied to film theory, this theory tends to explain film history and the evolution of film almost exclusively in terms of "Great Men", with outright geniuses, like the ones listed on the pages for directors. It neglects the efforts of crews, assistants and outside constraints. It could be described as the film history equivalent to the star system or the auteur theory