A bad link (or broken link) is an hyperlink on a web document (web page, image, etc.) from your web site that point to an another document (on the same site) that doesn't exist. This is a structural error in your web site that should be corrected as soon as possible. The "Requested document" is the document that could not be found and the "Referer" is the document where that document is linked.
Warning: the bad link report allow you only to identify the bad links that point to your web site (internal links). To identify external bad links, you will need a link verifier tool. see also Bad refered links.