Infrared heat is electromagnetic energy with a frequency that exists below the visibility level of the human eye in an electromagnetic spectrum. That is, human beings cannot see it but human bodies can detect it as heat. Infrared includes radio waves and X-rays.
The sun is our main source of radiant energy.
Think of walking outside on a sunny day. The air temperature is constant, but you immediately feel cooler when you walk into the shade.
A gas fired infra-red burner emulates the efficiency of the sun by generating radiant energy.