Hi Pineapple,
I am by no means an expert but the approach I am using requires me to be reflexive and reflective throughout my thesis and analysis. I think it's probably going to be about how you conducted the research, what went well/wrong, why it went well/wrong, what you would do differently and why. I would guess they want you to show how you have developed as a researcher. I think it will vary slightly depending on what kind of approach you've taken with your research. I think it might be worth pinging it past your sup just to check.
My thesis will have a section either in the conclusion or as an appendix discussing the use of reflection/reflexivity and the impact I had on my data as a dual insider/outsider. I'm writing the whole thing as a very open, confessional style. I hope that's somewhat helpful?