A codeplug is basically a program that's loaded into a radio and tells it what frequencies to transmit and receive, RF power output, carrier squelch/coded squelch, signalling modes, and whatever other features the specific radio might have you want enabled.

 

Back in the good old days, radios used crystals to control the frequencies, and sometimes had internal jumpers that were set for the various options. Some time later on, they moved the internal jumpers to a jack on the back of the radio. They configured plugs with wire jumpers in them that plugged into the jack to enable certain options. These became the original codeplugs. Later on, more things were controlled by these codeplugs, such as tone encode/decode selection.

As radios became microprocessor controlled, the external codeplugs moved inside the radio as programming information. The term has remained with us, although there's not much that still plugs in. The codeplug contains the operating frequencies, tone selections, timeout values, system IDs, etc. In some instances, there's even some parts of the program itself in the codeplug which defines a radio's personality. These days, the codeplug is just a relatively small binary file that's managed by the RSS program and transferred to, or from the radio via several proprietary means.