To translate in broad and inaccurate strokes, imagine if the universe gets drunk and takes random pot-shots every so often with a huge radioactive hyper death-ray*. The other day, the random shot was close enough to blasting us that we were able to see down the barrel.
To continue the bad translation, when a star collapses, it often sends out a wave of gamma radiation. If the star is spinning at a pretty good tick, though, that makes the wave funnel in like water going down a drain, prompting a blast coming out of either pole of the star's spinning. This blast can go a long, looong, loooooooong way. Any planets caught in the heft of the blast are basically super-microwaved on the side of impact -- air burns off, oceans boil away, all the air and ocean on the "safe" side gets hauled over to the blasted side like wax being carried along a wick. We're fairly sure a GRB was what prompted at least one of the times that 90%+ of all life on Earth suddenly died.
Kind of puts all your hypochondriac fears into perspective, no? Yesterday, we barely missed an event that would've killed half the planet in the span of a few minutes.
Oh, and since gamma rays move at pretty much light speed? We wouldn't even get a warning by way of a flash of visible light or anything; It'd just be, "Tra la la... *ZOT* ...thud".
* Hyper death being a fate +30% worse than death**
** Joke shamelessly stolen from the "Buzz Lightyear of Star Command" animated series.