Iridium Flares are bright lights in the sky caused by sunlight reflecting off a particular group of satellites named the Iridium satellites. As the satellite moves through the sky reflecting sunlight so it leaves a trail of light in a photograph.
This is the first iridium flare I have ever captured in a photograph, and it was by complete accident! I was photographing the Milky Way with Large and Small Magellanic Clouds in the southern hemisphere of the sky at the time, and just happened to nab the Iridium Flare too. Very happy!