A historical factor is any element of influence on a situation that is related to history. A historical factor could be anything from President Abraham Lincoln's Emancipation Proclamation to World War II as these things exert influence on many events.
A factor could be anything, including a result or situation, that contributed to another result or situation. The word "historical" is an adjective which describes something that is somehow related to history or previous events. Because adjectives describe nouns, and "historical" is an adjective describing the noun "factor," a "historical factor" would be describing an element contributing to a particular result that is of or relating to history.