The term Codeplug has been popularized in recent years because of the rise of DMR, a commercial digital voice mode that has been adapted to Ham Radio.
Put simply, it is the config file that can be used to program a radio. Some people use Chirp or other software to program their radio. A codeplug is simply that config file.
The term originated from Motorola from back in the day when there was a physical “plug” that was used to setup the radio. DMR was popularized by Motorola and the term Codeplug is common among Motorola equipment.
Since DMR has become popular people have been using the term Codeplug to represent any radio config.