It means that the photos have been moved or renamed or deleted outside of Lightroom. (Hint: don't do that)
To fix the problem, please follow these instructions:
thanks very much indeed.
I'm busy working through what is a very thorough book by Scott Kelby on Lightroom 5.
Evidently I took a wrong turn on the External Drive section.
Have a good evening.
My advice would be to completely ignore the parts of Kelby's book that explains organizing and file storage, where his advice IMHO is poor; and go straight to the part where he explains how to edit photos, where his advice is much better.