Radiant heat, also known as thermal radiation, is the transfer of electromagnetic radiation which describes the heat exchange of energy by photons. Radiant heat is a mechanism for heat transfer which does not require a medium in which it propagates (unlike convection and conduction). All substances above absolute zero have thermal energy, which means that the particles contained in them have some form of motion. This motion of the particles contributes to the temperature of the object, with objects of "ordinary" temperatures (less than 1000 Kelvin) emitting their radiant heat primarily in the infrared spectrum of light. The photons emitted by these moving charged particles will travel at the speed of light until they hit another particle, which absorbs its energy as kinetic energy. Interacting systems at different temperatures will do so by the exchange of radiant heat until they reach thermal equilibrium with one another. Although the exchange of photons doesn't stop at equilibrium, it cannot be noticed because of the identical temperature of the systems.
Radiant heat is present in everyone's daily lives. The fact that humans are even alive is because radiant heat from the Sun interacts with the Earth, driving important global cycles and allowing life to flourish.
When sitting outside on a hot day, the feeling of "hot" comes from the radiant heat on your body from the sun being more than the heat emitted due to your internal temperature, so your temperature will increase. In contrast, when sitting outside on a cold day you will feel "cold" because the heat transferred from the sun to you is less than the radiant heat you are giving off (along with convective heat transfer cooling you off). When the input heat matches the output heat of a person's body, their temperature remains constant which happens to be about 37°C. This balance is achieved in part by the evaporation of sweat from your body.
The Earth's temperature balance operates on the same principles of radiant heat transfer that human bodies do. Earth's energy flows keep the Earth almost totally in thermodynamic equilibrium so that its temperature remains constant, caused by the Earth's energy budget. However increases in greenhouse gases like carbon dioxide and methane are leading to slightly less heat being radiated into space than the amount of energy coming in. This imbalance is why the Earth is warming, which is the global warming part of climate change.
Since all objects emit radiant energy, it brings up the question of how exactly this energy is given off. Specifically, how much of it and at what rate is this energy given off? This is all explained by the Stefan-Boltzmann law, which says that this heat transfer depends only on the temperature of the body (which must be idealized as blackbodies). Along with this heat transfer law is Wien's Law, which describes the maximum intensity of light a body at a certain temperature will emit. This explains why as an object gets hotter and hotter, it goes from invisible, to red, to blue, and finally to white-hot, which corresponds to the decreasing wavelengths of this higher energy light. See blackbody radiation for more details on how temperature relates to objects and their radiant heat.
Figure 2: A picture taken in the visible spectrum is possible due to the light in the room reflecting off of the man. If those lights were turned off, he would be invisible due to his low temperature.
Figure 3: A picture taken in the infrared spectrum is possible because the radiant heat given off by the man is in this spectrum, and can be absorbed by the camera. Notice that some objects are transparent in infrared but opaque in visible spectrums (the bag) and vice versa (his glasses).