It means iPhoto can't find the full-scale image that was tied to the thumbnail. This is nearly always the result of people messing around in the iPhoto Library file directly when they really, really, shouldn't do that.
Without knowing what you might have done, it's hard to suggest anything. If you are absolutely 100 percent sure you have never messed with the iPhoto Library package, you might try restarting iPhoto with the command and option keys held down, which will bring up a repair dialogue. Check every option and give it time to do its thing. That might help.
If you've been moving "originals" around or out of the iPhoto Library file, then you've messed up the database and much more extensive repair work will need to be done to fix things.