A solar storm is an event that happens when sun flares are sent out from the sun due to a massive explosion in a star's (such as the Sun's) atmosphere, that sends out specific amounts of energy. Sun flares cause the plasma to heat up to several million degrees and accellerates electrons, protons and heavy ions to near the speed of light which causes the solar storms to errupt.
A solar storm happened on a planet which hosted the Bank of Karabraxos, the most secure bank in the universe, and wiped out the entire bank. The Doctor received a phone call from an elderly and dying Madame Karabraxos, telling him about her many regrets and sending him an Clara Oswald on a mission to rescue The Teller (Time Heist) and it's mate, humanoid creatures with the ability to scan and liquefy people's brains, with the aid of a shape-shifting mutant human named Saibra and an augmented bank robber called Psi before the storm wipes out the planet and everything on it. (TV: Time Heist)