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It means simply that the browser did not pass a referrer to the tracking system. These days there are a lot of reasons for no referrer being passed. Some are:
1. There actually was no referrer, i.e., somebody typed in the address, or clicked on a link in an e-mail, or clicked on a saved bookmark. Also, spiders and bots from search engines may be working from their own list of URLs (from a previous crawl) rather than following links.
[IE is, to the best of my knowledge, the worst culprit for not passing referrers under various conditions. Firefox, Opera, etc behave differently.]
3. Some spiders/bots/crawlers suppress the referrer.
4. Not in your case, but sometimes with log analysis, recording the referrer field has not been turned on in the server. In that case you'd see 0% named referrers.
Somewhere else on this forum is an extensive and knowledgeable discussion of this. I've just touched on it.