Check Marks in Song List for .......

1. If you turn off a check mark (the tick) against a song in iTunes, iTunes will not play the song unless you specifically select it. This applies even in normal play mode. In other words, if you turn off the check mark for track six of a nine track album, and then play the album, iTunes will not play track six.
2. There is an option when sycnhronising an iPod, iPhone or iPad with iTunes, to Sync only ticked songs and videos. If you select this option, any song that is not ticked (no check mark) will not be added to the devcies.

In the first scenario, this is a useful way to prevent a song playing at an inappropriate time. For example, if you wish to use your iTunes Library to play a playlist at a family event, but without the explicit songs in the list, you could simply turn off the check mark to prevent them playing, without having to edit the playlist.

In the seocond scenario, when syncing with a portable device, if I want to remove certain songs from my iPod but keep them in my iTunes Library, I simply untick them in my Library and then Sync my iPod with iTunes. It's a quick way of removing seasonal songs from my iPod until the season comes round again and I want to put them back on. Christmas songs for example. Removing the check mark also stops them playing in my iTunes Library as well.