A solar flare is basically a large release of energy from the Sun.  The flare is often followed by a large coronnal mass ejection, or release of a large particle cloud comprised of electrons, ion, and atoms.  

These large formations typically reach the earth within a couple days (short notice!) and with direct impact to an electronically-dependent society, have the potential to wreak havoc on an unprotected electric grid.

The world has encountered many such Solar Flares over the centuries, but fortunately the developed (electronically dependent) nations have not encountered a direct hit (due in part to the relatively short existence of electricity on this planet). In general, concerns are mounting in the scientific community that a solar superstorm could be a devastating phenomenon to our society as it stands.

Please see below for more relevant references:

Wikipedia-

On July 23, 2012, a massive, and potentially damaging, solar superstorm (solar flare, coronal mass ejection, solar EMP) barely missed Earth, according to NASA. There is an estimated 12% chance of a similar event occurring between 2012 and 2022.