um...where the files are stored (the physical mp3s or aacs, generally in the itunes folder in yhour music folder). if you look at them, then you can see which have a little lock on it and which don't. The ones with the lock are the ones you can't access.
Why? this is generally a message you get when someone has downloaded (*bought) music from itunes and someone else is listening to it. If these are all your legal downloads, then chipper
, you need to authenticate each one on your computer. If not :-( bad, don't steal music. If it was a communal effort, ask who ever's it is, for their password...but it's also their itunes password, so that may not fly.