Dreams that vivid usually have to do with something that worries you or intrigues you in real life, even if you don't have it totally clear. That "I know who it is but I don't recognize them" thing you're describing is typical of when your mind is trying to process someone important or an aspect of yourself that you still don't fully understand. Sometimes it happens when you're expecting communication with someone, or when there's something unresolved in your head about a relationship or situation. The thing is, if the dream left you with a strong feeling, your subconscious was probably sending you a message about something you need to pay attention to.