Linked files are external files such as images or graphics, that are not physically embedded in the main document, but are referenced in it.
When adding images or graphics to your artwork, it is possible to either import your file completely into your artwork, or simply to create a reference or “hot link” to it.
It can sometimes be preferable to do this, however when it comes to ultimately printing your file, it is important to make sure that those linked files are correctly referenced and included, else they will fail to print entirely, leaving you a decidedly empty space in your resulting print.
While we do everything we can to spot these issues and get them resolved, it can sometimes be hard to recognise when something that should be there, is not.