They show the detectors that picked up the strikes. I don't know if a single detector can determine distance, or if the strike location is triangulated using angles from multiple detectors.

[edit] Oh, the detectors are quite simple. They use GPS for position and time and report time-of-arrival of the elctromagnetic pulses. The server then uses that information to figure out where the strikes occurred.

http://www.blitzortung.org/Webpages/index.php?lang=en&page=3