An indication that a
message has been received and understood.
Based on the set of Ten Signals or “ten codes” developed for radio communication by the Association of Public-Safety Communications Officials-International. The codes were popularised by TV show Highway Patrol (US, 1955-1959), which regularly featured their use. Hit movies like Smokey and the Bandit (1977) and Convoy (1978) only helped cement CB radio slang in the public consciousness. Ten four is the only one of the ten codes still in (relatively) common use (although code ten-twenty, meaning "what's your location" also survives in phrases like "what's your twenty?").