Infrared, known as I/R, is a form of light, which is almost invisible to the human eye. I/R is used with night vision or other equipment, which is I/R sensitive. Almost all night vision units are equipped with built in Infrared Lighting. I/R is used in total darkness, or when there is too little light for the light amplifier in the night vision unit to work with, such as caves and warehouses etc.
In such a situation, the I/R is activated, and the nightvision unit now has a lightsource to amplify. You are now able to see as if you where in daylight, even if you are in total darkness.