What does it mean to dream about someone who is dead but your dream that person is dead and you know it but other people dont?

If the person in this dream has recently died, the dream might be part of the grieving process. The dream represents the dreamer's difficulty in accepting the reality of the person's death. In the dream, "you" are the intellect, which understands that the person is dead, and "other people" represent the emotions, memories, and other aspects of the Self.