A Score Scroll is an item used with the Composition skill to write music that can be played on an instrument. Each can only be written on once, and the amount of music that can be written depends on the player's Composition skill. One writes songs onto score scrolls by using a music language called MML, Music Markup Language. Once it has been written on, it cannot be edited and can be used up to 100 times before it breaks. The writer has the option to 'close' the score scroll up writing, to prevent other people from trying to copy the scroll, though the scroll only has limited display space, and the whole song may not be seen in the first place. Score Scrolls can be found in most General Shops around Erinn, and are often sold in 4 different colors.
Some pre-written score scrolls can be purchased from NPCs, such as Nele, which are good for people to practice with who don't have composing, or a friend to compose for them. They are just short ditties, and each displayed score is written for a specific instrument or instrument type.
Please see the MML page for more information on writing music.