I found this on one of the flickr help forums:
"Unknown Source" is what we used to call "Direct Traffic".
To generate the stats flickr relies on your browser telling us what page you were on before you followed the link to the flickr photo. Most of the time this works fine, but sometimes:
. someone is clicking on a link in their email program or an instant message, where there is no referring "page" to link to
. some people set their computers up to deliberately hide this information
. sometimes someone directly enters the address into their browser, or uses a bookmark
In all these cases (and a few others), we don't actually know where someone came from, so we show "unknown source".