What does it mean if a song is checked in my iTunes library?

[TH55]

And why are certain songs automatically checked that I didn't check?

 

[Mrbobb]

In a configuration screen, it asks, Do u want to sync all songs that are checked? If u leave this checkbox blank it means ALL songs are automatically synced and when u add a song the individual checkboxes are let blank because it won't matter.

If u say YES to the above then no need to repeat the obvious, and when new songs are added, iTunes defaults to YES SYNC THIS SONG until u manually say otherwise.

 

[Ann P]

If there is enough room, by default your iPod is set up to be an exact copy of your iTunes library.

If there are some songs in your library that you don't want on your iPod, you can uncheck them in your iTunes library list, and choose the Sync only checked songs and videos. This will remove those particular songs from your iPod.

I always like to recommend this beginner's guide. It gives a good overview of all your sync options:
http://www.ilounge.com/index.php/articles/comments/the-beginners-guide-to-filling-your-ipod/