"10-4" is a code used in radio communication to indicate acknowledgment or understanding of a message. It is part of the ten-codes system, which was developed for clear and concise communication, especially among law enforcement and emergency services. When someone says "10-4," it means they have received and understood the information conveyed. The use of "10-4" has also permeated popular culture, often associated with CB radio communication among truck drivers and hobbyists. It serves as a quick way to confirm that a message has been received without needing to repeat the entire conversation, enhancing efficiency in communication.