The answer you found was not incorrect. It just depends on how you interpret what the term "role" means as you write on this topic. When something is significant, that means it is important for some reason. Characters can be significant in all sorts of ways. Let's look at a few.
A character can be significant because he or she represents some concept or theme. Who, in To Kill A Mockingbird, for example, is the character who represents the mockingbird? Some people think that Boo Radley represents the mockingbird. Can you support that idea?
A character can be significant because he or she acts as a foil for another character, for example, a protagonist and an antagonist might appear in a story to balance one another and create a conflict. Atticus is one protagonist in the story. Who might be an antagonist?
A character might be significant in a story because he or she is the hero of the story. Who are the heroes in this novel? Boo Radley comes to mind for this role, as does Atticus.
If you think of the character's role in a story and ask what makes that role important to the story, you will be right on track. Remember that whatever the significance is that you find, you will want to support that with evidence from the story itself.