I'm doing a large research paper, what exactly does it mean it has to be written in the 3rd person?
This is an important question. It is first best to define what it means to write in the first person and third person.
Writing in the first person entails using the pronoun, "I". It is to write something like this: "I believe that Augustus Caesar was corrupt, because he took on absolute power." The assumption here is that you are giving your opinion of what you feel is correct.
When we write in the third person, we need to take out the pronoun, "I". We can simply write, "Augustus Caesar was corrupt, because he took on absolute power." To make things more sophisticated, you would have to give reasons. The assumption here is that you are allowing the evidence to speak for itself. You opinions matter less than the evidence.
As papers become more sophisticated, it is important to stay away from the first person. You want to make your paper as objective as possible with lots of evidence from original sources.