That mean that the file is missing but still in the list or corrupted.
It also may be encoded in a format that Real Player don't suport.
Check that the file for those movies are still there and intact.
Try playing them using some other media player, like Winamp, VLC or IrfanVirew. All 3 are available for free. All 3 support more formats and encodings than Real Player.
Electro
March 2012