This message goes back for a number of Nero 5.5 releases. It's indicating that your source drive had trouble reading the CD with the source data, though it was able to recover from the read error. It warns you as it's possible the data written to the CD could be corrupted due to the read problems, or you coulg get a buffer underrun during the write due to read problems, if it's not able to recover from the read error fast enough.
In most cases you can simply ignore it, but you should be aware of it if your CD does turn out to not work after it's been created.
Most cases, cleaning the source CD first resolves this issue.
But it can occur with source data from a Hard Drive as well, though very rarely.