If the other person is doing something, like cutting vegetables or watching for something out of a window. You come in and start talking. They don't turn around but continue the conversation: I would say they are saying, I am busy and this is more important, but I will take your conversation into account.
If they turn around while you are speaking: They are concealing their facial reactions from you.
If you walk up to them and talk, they are doing nothing, but don't turn around and talk back to you: Your conversation is not important and they want you to go away.
If you have a conversation and they turn their back after the conversation has finished: The conversation is over, they don't wish to discuss it anymore, they want you to go away.
I can't think of other reasons the other person would be facing the other way.
You should look up symbolic interactionism.