A dead person whom you knew (or knew of) in real life can represent that person, or your memory of that person.
Seeing such a person in a dream can mean they are particularly on your mind right now, perhaps because you've been thinking about them or would like to interact with them, because a significant date (birthday, anniversary, etc.) is near, because you're curious about how they are or what they might think about something in your life right now, etc.