There a couple of different theories to your question.
First, some believe that we dream of people we have seen, even if it is in passing. For example you are at a crowded mall, you might not of noticed someone, but your subconscious mind did. Then when you are dreaming, the subconscious mind brings the people it notices to your dream.
Second, some believe that when we dream of people we have never seen, they are people we know from a past life.
If you want, you can spend time with these dreams. Retell them using your five senses (touch, sound, taste, see and smell) as well as, the emotions invoked through out the dream. This will help trigger dream content as well as, meaning. Also, how do you feel around this strangers (are they really strangers or do they seem familiar?) Do you feel as if you know them? Another technique is to talk to them. It might feel weird at first but let your imagination flow freely and don’t censor yourself. Imagination is another way our subconscious communicates with our conscious mind.