Currently, if you have a properly coded journal style, utitlities have the ability to find an RSS feed of your journal simply by visiting your journal. In the source of the page is a tag:
<link title="RSS" href="http://www.livejournal.com/users/crschmidt/rss" rel="alternate" type="application/rss+xml" />
This tag tells any smart passers by that there is rss-xml information awaiting delivery at the URL mentioned. A tool could then go, gather the information for my entries, and have everything that it needs about the content stored by my journal. It would have a list of the most recent entries, who the author was, and more.
FOAF takes this concept a step further. Because it's possible to describe the person who owns the journal, you can find out a wide variety of information about the journal and author in question. In order to tell tools that integrate this type of data where your FOAF file is, LiveJournal will add this line to the <head> section of your journal:
<link title="FOAF" href="http://www.livejournal.com/users/crschmidt/data/foaf" rel="meta" type="application/rdf+xml" />
Because FOAF tools list a large number of people, it's best to identify who you are in the FOAF file. LiveJournal does this by adding the following line:
<meta name="foaf:maker" content="foaf:mbox_sha1sum '1ba3bffdd13a136d38b77542f2e26fd1dc0042f2' />
This string matches your FOAF file, where it will be generated based on your email address. This will tell FOAF tools which foaf:person is you specifically, rather than anyone else.
For more information on the foaf:maker tag, you can see http://usefulinc.com/foaf/iMadeThis . A good example of a tool which uses these links is Dashboard.
That's how LiveJournal tells web crawling type tools how to find your FOAF file, and to let them know that you were the one who made it.