Scientists believe the source of the burst was the deterioration of a neutron star, but mainly remains largely a mystery. I'm inclined to lean more towards the scientists on this one. The more fringe groups side with aliens, but this seems unlikely. A burst of this magnitude, if it was a signal, would probably have a more distinct pattern conforming more towards the cryptographical than the random, which this seems to be.
The source is 5.5 billions light years away. In addition to this, nine other radio bursts have been detected since 2007.
If you're worried, don't be. These bursts are no threat to life on earth and probably won't even be picked up on your car radio.