Solar Flares, or Solar Eruptions, are huge explosions on the surface of the sun. A mushroom of cooled plasma jettisons large amounts of solar material into space when this happens. Flares are not uncommon. Space observations recorded about 70 in 2010.
It is no secrect that flares affect HF-radio wave propagation. The increased energy, when it hits the earths magnetic atmosphere, causes higher noise levels and weakers signals. Long range contacts become more difficult, if not impossible.
Every once in a while however an extreme explosion occurs on the sun. These flares may be up to 10 times more powerful than 'normal' eruptions. With the sun's 11-year cycle of activity, driven by tangled surface magnetic fields, hitting its maximum in late 2013 or early 2014 extreme flares are something to take into account. Some nasty magnetic storms are predicted.