The explosion was a gamma-ray burst, or GRB, a type of event that's the brightest we know about in the universe. During a supernova, in which a massive star collapses into a black hole, neutron star, or quark star, sometimes a GRB is emitted. Nobody's exactly sure how GRBs happen, but they're observed during a supernova and consist of a tightly focused, narrow beam of radiation, moving at speeds about as close as you can get to the speed of light without exceeding it. Imagine compressing a weird space-apple from all sides until a jet of space-apple juice explodes out of a tiny point before the apple turns into a black apple hole. (This is not a perfect analogy.)