Answer:
They're spots on the sun that are cooler than the rest of the surface. Take the word "cool" with a pinch of salt though, they're still about 4500 degrees hot (general surface temperature of the sun is 6000 degrees). They're also a manifestation of magnetic activity, and this causes solar flares, which is a mass-release of energy from the sun, sending electrons and protons hurtling though space at near the speed of light.