My professor said my outline is 'too vague' and I need to write a research paper with more depth. What does that even mean?

[EricaWilter]

I thought I was doing everything right. I turned in a detailed outline with my research question, my main arguments, and the sources I planned to use. My professor handed it back with a single comment: 'This is too vague. Needs more analytical depth.' I stared at those words for an hour. What does 'depth' look like on paper? I'm a sophomore, and I feel like I'm missing some secret academic code. How do you move from summarizing what other people said to actually adding something new? I don't feel smart enough to have 'original ideas' about a historical event that happened 200 years ago. I'm scared to even start writing because I don't know what she wants.