Radiant energy is the physical energy resulting from electromagnetic radiation, usually observed as it radiates from a source into the surrounding environment.
Radiant energy sources include the entire electromagnetic radiation spectrum, including gamma rays, x-rays, radio frequencies, microwaves, light and heat. Often, radiant energy is used to describe the type of particle in question, but it is actually the energy carried by the particle. Radiant energy is not the photon itself, for example, but one can see the energy as light and feel it as heat as the photons it is associated with arrive on your skin.
Radiant energy travels in wave forms. Types of radiation with higher frequency are higher energy. As the source material is excited, the radiation of energy is increased. One can see this in effect with the turning up of a dimmer switch: As the power is increased, more light is emitted from the bulb, be it LED or incandescent. This effect is the result of more photons are radiating off in all directions.
Naturally occurring sources of radiant energy such as the sun can be converted into usable electricity with solar power or used as passive solar. Nuclear power plants use radiation to boil water driving steam turbines. Radiant energy and its behaviors have been harnessed in many inventions from the light bulb to the large hadron collider.
See a video introduction to radiant energy: