Glossary
Attorney-client privilege
A legal rule that keeps confidential communications between you and your lawyer (made for the purpose of getting legal advice) out of evidence. The other side can't force you or your lawyer to reveal what was said.
Attorney-client privilege exists so people can be honest with their lawyers without fear that what they say will end up in court. It covers communications: what you said to your lawyer, what they said back: for the purpose of legal advice.
The privilege can be waived, accidentally or on purpose. Forwarding a privileged email to a friend, discussing it on a non-privileged platform, or putting it in a public filing all destroy the privilege as to that communication.
The privilege belongs to the client, not the lawyer. Only the client can waive it. After the client dies, in most states the privilege survives: though there are exceptions.