We really have no handle on what they are. Scientists are highly skeptical of such discoveries … [that] all of the bursts up until now had been discovered by the Parkes telescope was a cause of concern. Now, with the discovery of a burst from Arecibo, we are more confident that FRBs are astrophysical phenomena, and discovering and classifying them should be a priority of radio astronomical observatories in the future.
Until we're able to uncover more of these FRBs and properly study them, though, scientists are at a loss for what could be causing them. Some proposed explanations have included evaporating black holes, mergers of neutron stars, and flares from extremely powerful neutron stars called magnetars. And as Duncan Lorimer, one of the astronomers who uncovered the first set of FRBs told NPR:
There's even been discussions in the literature about signatures from extraterrestrial civilizations.
It may be quite a while before we get any definitive answers, though. The two radio telescopes that detected the waves suffer from tunnel vision, which means that since our view is so limited, any FRBs we find in the future will largely be thanks to luck. But as Cornell astronomer James Cordes also said to NPR, "The nice things about this in the current stage is that we really don't know what these bursts are caused by. And so the sky's the limit in some respects."
Or to put that in layman's terms: