History is the version of past events that people have decided to agree upon

What does this quote mean

It means that the people that write the history books or articles or whatever else is used have decided to word what they're writing down and documenting as "important," and how sometimes that the limitations and reality of situations of the past change from each person's decision of either how to describe it, give it meaning, take meaning, and so on. It's saying that whoever is in charge of documenting things and deciding what goes in and what doesn't will be the version that future generations of people will study and debate, wether it was real or not.