I'm not an expert but I can try to explain it with my own words. A fartlek run is just a run with a combination of fast and slow run. In this case it was 15" fast and then 15" slow but it can be 40"/40" of even long fartlek like 1 km at a fast pace and then an active rest of 2 or 3 minutes.
And I just looked on Wikipedia and apparently fartlek means speed play in Swedish!