Yeah, it's confusing. It's a shame it doesn't explain it right there on the main page
Paste an email with full headers (we need the 'Received' lines -- we don't need your email addresses, digg). The app will (we hope) trace the path your email message took as it passed through various servers, on Google maps.
It only takes the last 4 or so servers and is limited by hostip's ability to provide a lat/long coordinate for the given IP.