A solar flare is a spontaneous release of energy on the Sun’s surface. The Sun’s surface contains large magnetic loops. These loops, solar prominences, occasionally collide and short-circuit each other. This is what causes solar flares. These flares release vast amounts of high-energy particles and gases and are incredibly hot (they range from 3.6 million to 24 million °F). Solar flares do not last for a very long time, they tend to be only a few minutes long.
The main concern humans have with solar flares is the material that they throw into space. This materiel is dangerous to equipment, which can be erroded by the flares, but it is even more dangerous to astronauts, who can be exposed to lethal levels of radiation.