Radiant energy is the energy of electromagnetic waves. It is a form of energy that can travel through space. For example, we receive the heat from the sun, which is located very far from the earth via radiation. The sun's heat is not transmitted through any solid medium, but through a vacuum. This is possible by electromagnetic waves.
Before we go any further, let us understand what electromagnetic waves are.
Each time static energy from electric and magnetic force come together, they induce an electric field around them.
An example of electric static force is the shock you get when you hold a metal door knob.
An example of a magnetic force is the pull that attracts metals to the magnet.
Now, the electrical field induced causes waves, called electromagnetic waves, and they can travel through a vacuum (air), particles or solids. These waves resemble the ripple (mechanical) waves you see when you drop a rock into a swimming pool, but with electromagnetic waves, you do not see them, but you often can see the effect of it.
The energy in the electromagnetic waves is what we call radiant energy.
There are different kinds of electromagnetic waves and all of them have different wavelengths, properties, frequencies and power, and all interact with matter differently. The entire wave system from the lowest frequency to the highest frequency is known as the electromagnetic spectrum. The shorter the wavelength, the higher its frequency and vice versa. White light, for example, is a form of radiant energy, and its frequency forms a tiny bit of the entire electromagnetic spectrum.
In the illustration above, you will see the different radiant energy levels represented by their wavelengths.
When radiant energy comes into contact with matter, it changes the properties of that matter. For example, when micro-waves (which forms part of the entire spectrum) are set off in a microwave oven, the water molecules in the food are charged and caused to vibrate billions of times per second, generating heat, that causes the food to cook. The microwave oven works with the concept of radiant energy (electromagnetic waves).