Why DMR radios are not tuned like analog rigs; nearly everything is preprogrammed. A code plug is the configuration file, built on a computer with the manufacturer's programming software and uploaded to the radio, that holds channels, repeater frequencies, color codes, time slots, talkgroup IDs, contacts and zones. Without a current code plug the radio cannot key up the digital repeaters or join the right talkgroups.
Watch out The CODEC answer describes the AMBE+2 vocoder that digitizes your voice, which is hardware/firmware inside the radio and has nothing to do with the configuration file.
Code plug = the radio's settings file, not a cable, not software, not a codec.