Religious folks will tell you it means they need your prayers to help them through purgatory.
Some will say its them reaching out to you as some sort of sign.
Realistically it means nothing. You brain just goes into random thoughts when you sleep and your mind is trying to make them connect so you end up with random stories.
If you are talking about what it means for the future....then it doesnt indicate shyt. Dreams are a recollection and organization of past events and feelings.
I think sometimes it means you miss them. And sometimes it means they visited you. You will know the difference.
It means you miss him/her.