These plugins allow Winamp to notify mIRC of what track you are playing. The user can configure what command the plugin sends to mIRC, e.g. "/set %currentsong %s", where %s is replaced by the name of the current song, as in the ID3v tag.
By putting a "|" (command separation character) and mIRC-commands in the ID3v-tag and tricking the user into playing the mp3, an attacker can take full control over a victims computer.
Setting the songname in the ID3v tag of an mp3 to "t | /run notepad" and playing it with winamp, with the plugin installed and enabled and mIRC running, would start notepad.