I was taught in class today about this theory and I could not understand it thoroughly. So, I seek help from this great subreddit topic.
PS. I'm not English native speaker. So, if there's anything wrong with my English, I do apologize in advances.
There is always more to be said, but you may be interested in these previous answers:
When and why did the “Great Man” theory of history fall out of favor amongst historians? by /u/TenMinuteHistory
Monday Methods: On Great Man Theory, Trends and Forces Theory, Capitalism, Socialism and a general history of how we write history by /u/commiespaceinvader