The Legendary Score is a book of music that contains magical notes and originally contained "The Melody of Happiness". When sung by the right person/fairy it has the ability to bring hope and peace to all worlds but the notes can be rearranged to have other effects such as when Mephisto used it to create the "Melody of Sadness". The notes it contains also seem to be alive and Aphrodite comments to Hummy if the song is not sung just right the notes could escape the book.
These are the notes, that Hummy and Mephisto are trying to find, as to complete the Score, and perform the Melody of Happiness / Melody of Sadness once they are returned to the book. Each of the Fairy Tones, are responsible of collecting these notes and serve to store them in the absence of the book itself.
When they hide, they are usually attached to an object. Mephisto and his minions, can corrupt the notes by calling them, to create Negatones.