Radiation is energy transmitted in waves or a stream of particles. The first thing you think of as radiation is probably X-rays, but what cooks your food in the microwave oven is also radiation.
Radiation has some important medical uses, especially in the treatment of cancer, as "radiation therapy." It’s also associated with such horrors as "radiation sickness" — observed in people exposed to high levels of radiation, as in the aftermath of the 1986 explosion at the Chernobyl nuclear reactor, in the Ukraine. You can also use the word radiation, in more benign contexts, to describe something spreading out from a central point, as the radiation of warmth from your fireplace.