Answer:
Just means that an artist/writer/filmmaker/propagandist can supplement or alter facts of a pre-existing story, a quote, grammatical rules, and just about anything else you can imagine for a artistic or dramatic purpose. For example the recent movie Bohemian Rhapsody is “based” on the life stories of the rock band Queen. That doesn’t mean that every detail in the movie is accurate or that the personalities and interpersonal dynamics were presented in an entirely authentic way or just the way they occurred. The writers, directors, producers, and actors all took dramatic license to create a 2 hour movie that captured their artistic interpretation of the essence of the story. Also to make it entertaining to watch so that you would pay to see it. The movie isn’t how it actually happened. That is why they always say “based on a true story” not “the facts as they actually occurred”. The degree to which some of those stories are “based” on the actual events varies significantly. Sometimes to the point of being unrecognizable. Every film/art/writing/book etc. has the creator’s agenda behind it. Sometimes that agenda is to make you think, sometimes to entertain you, sometimes it is to pander to your preconceived notions, and every once in a while it is to deceive you with propaganda.
