what does the circle with the exclamation point mean next to a song title?

Hello, I have an ipod touch fifth generetion 64 gig capacity. Some of my songs in my itunes library have a circle with an exclamation mark in its center next to the song title. What does this mean? The songs with this symbol won't play in itunes and niether will they sync with my ipod when connected to my windows computer running Vista. Any help will be greatly appreciated.

J.S.