The origins of the phrase "10-4" are unknown, but it is believed to have originated with law enforcement officers in the 1930s or 40s. Once the phrase caught on, it became a popular choice because it kept radio conversations short and to the point.
In terms of radio communication, 10-4 simply means "okay" or "I received your message." Ten-four is part of a number of codes called 10 codes, and these codes are still used today.
There are 200 10 codes, and each one has a specific meaning. Another popular 10 code is "10-20." This phrase is a way of asking for a person's current location.