It's a filter expression again, as you probably have guessed yourself.
$list() is a undocumented function in Mp3tag. I have no idea why it isn't mentioned in the help files. Here is a very good explanation from DetlevD:
Lyrics aus Datei importieren?
The $regexp() function around it deletes tge values of the fields and the <<< >>> seperator and leaves only the blank field names wihth a single space after each name.
Use the Tag-Filename Converter if you want to see a preview of the functions (do not press OK):
$list(, <<<,>>>)
$regexp($list(, <<<,>>>),<<<.+?>>>,)