Message authentication

Message authentication is like a secret club. Whenever two people want to talk, the first person (let's call them Person A) tells Person B a secret password. Only Person A and Person B know the secret password, and they use it to make sure nobody else can listen in. Person B then uses the secret password to prove to Person A that he or she is really who they say they are. This helps keep their conversation private, so that nobody else can pretend to be Person B and steal their messages.