never heard that word in particular, but meta- means "beyond" kind of... like "metacognition" means thinking about thinking... kind of a thinking beyond thinking...
so from that, I would venture to say that it's music that supercedes tonality. It goes outside the realm of tonality, but it still in it's own world the music is tonal. Tonal in the sense that it follows it's own rules of tonality.
...
just a guess, but maybe someone can give a better definition.