Funny you're having trouble "searching" out the answer; it's how you go "backwards" from where you are in a search; so if you follow a link to a link to a link (or on a directory if you go down levels) it keeps track of how you got to where you go, so you can go back to where you were. I'd guess it's a kind of structural "safety" so if the blog software crashes, one can recreate it and not lose the whole thing. It's part of the internal structure of the blog I believe. Multiple ways to get somewhere have to have breadcrumbs so one can get back