This often happens because of bad sectors, but can also be due to power loss while writing data and many other things. Basically, it's data that can be read, but it isn't completely intact so it might not be meaningful or usable. If the sector is damaged the data may have been corrupted or misread. If you're unlucky, you may find this happened to you. In that case, you'll likely need to use some more tools to fix your output file from DDRescue-GUI before you can use it.