Laura:
Unless you have specific reasons for turning off a check mark, leave them all on because there is an option in iTunes which will prevent unchecked songs from going onto your devices.
Reasons for removing the check mark against a song are:
- to stop it from playing in iTunes. When playing songs in your iTunes Library (on your computer) an unchecked song will not be played. Even if you are playing a Playlist, any unchecked song in that Playlist will be skipped (not played) if it is unchecked.
- when Syncing your music to your iPod, iPhone etc., any song that is not checked will not be added to the device, even if it's in a Playlist you are putting onto the device. (There is an option to override this feature.)
Why would anyone want this?
- You may not want to listen to seasonal songs outside of that season, but you want to keep them in your Library so that you can check them at the appropriate season time. Or, a song may in fact be a speech track, such as an interview with band members. Unchecking it means it will not be played when using iTunes, unless you specifically select it.
- Unchecking a song can be a quick and easy way to remove a song from your device. I use a one-star rating for any song I don't like. Every now and again I check my "1-star Smart Playlist" for 1-star songs and uncheck them. As a consequence, they are then removed from my iPod. I find it a quick and easy way to remove songs that I don't like from my iPod.
If you or anyone else doesn't understand why check marks are used, perhaps it is because you have no need of them. In which case, leave them on, because if you turn off a check mark, it will not be played, ever, unless you specifically select it.