Infrared (IR) is often called "invisible light" because the human eye cannot see it. Infrared light occurs beyond what is known as the "visible light spectrum". However, it can be captured with digital cameras that have been specifically modified to "see" IR light.
There are often surprises when shooting IR pictures depending on the object or subject being photographed and how it interacts with the infrared light. Does it absorb light, reflect light; does it direct IR light or scatter it? In my IR pictures, notice how often leaves and grass are captured as white and blue sky becomes black. Enjoy!