A person you don't know can represent many things, such as:
A real person you actually know, or a particular person's role in your life
A characteristic or general type of person
A type of person you'd like (or wouldn't like) to know
A general threat or fear (not a person)
A problem, challenge, issue, to-do list item, etc.
A religious figure or helper
For clues about what the person represents, consider their characteristics, what they're doing and how, what stands out about them, and your emotional reaction to their presence.