In English, a "reflection" effectively means a short summary. So your teacher probably wants you to explain what you did well, what you did poorly, what you can improve, etc.

Here's an example: I thought that my essay/assignment/report about ................... was quite well written/poorly written because I used/did not use enough adjectives to describe.............. Another thing I could have done was to check my grammar and spelling before submitting my writing. I notice that I misspelled the following words[ insert the mistakes] Also I left out the definite article a few times in paragragh one. Add anything else you noticed about your work. Next time, I will plan my writing in more detail and spell check it.