What does "national treasure" mean?

Native speaker’s answer
Rebecca
In the formal sense, "national treasure" is used to describe things or items of cultural or historical importance. For example, it might be an artifact like an ancient vase or the like. In the informal sense, it can be used to describe things (or people) that are loved and welcomed by the whole nation. For example, the Simpsons is one of the most well-loved and recognized shows in American television history. This is why Rick refers to the Simpsons family as a "national treasure" in this video. Ex: Steve Irwin and his family are the national treasure of Australia. Ex: Our country keeps our national treasures in a museum with high security.