That is usually when an attorney and client have a discussion. Or maybe they wrote a letter to each other. Either way, when something is privileged, it means that the other side cannot obtain information about it.
Let's say I'm questioning a doctor at his pre-trial question and answer session, also known as a deposition.
I could not ask the following question:
"Doctor, I noticed you were just outside talking to your lawyer. I want you to tell me what she told you..."
"Objection, privileged!" is the response I'd get from the doctor's attorney.
That is true. Conversations between attorney and client are privileged and will not be disclosed.
As much as I'd like to learn what they talked about, I cannot legally obtain that information. Those discussions are confidential and even if I overheard some of what they said in private, I still could not ask the question and would not get an answer from the witness.