Radiation happens when heat moves as energy waves, called infrared waves, directly from its source to something else. This is how the heat from the Sun gets to Earth. In fact, all hot things radiate heat to cooler things. When the heat waves hits the cooler thing, they make the molecules of the cooler object speed up. When the molecules of that object speed up, the object becomes hotter.
Infrared waves are part of a spectrum of energy waves known as the electromagnetic spectrum.
The electromagnetic spectrum includes all kinds of energy that can travel in waves, including light, heat, x-rays, radio waves, ultraviolet waves and microwaves. All of these kinds of waves contain a lot of energy. Also, they can all travel through deep space. That's why we can see sunlight from stars billions of light years away. The light from them radiates to us.