Narrative police reports mean that you are writing what happened in the form of a story.
A "narrative" is a story. When an investigator writes a report, he or she is basing it on personal experience. It is important for the people reading the report to understand that you were the person who saw what you saw, and wrote the report. This is why the report should usually be in first person, using first person personal pronouns like I, me, we, and us. It is important that the investigator just tell what happened, and not color the report with extra information such as conjecture or guesses.