Problem Copying Files to CD

I just bought a CD RW to use for backing up my files with Windows XP. I have copied most of the files over to disk, but there are some others I would like to copy. The problem is that all the sudden, when I try to copy a file to the disk, I get the message that the files on the disk are “read only” and that files may not be added. Something seems different from when I was moving the files before. I know the disk is not full, because the total of the size of the files is less than half the capaciy of the disk. What does it mean “read only”, and how did the files become “read only” files? Also, is there a way around this situation?