Visible: Visible light typically refers to light that human eyes can actually see. This is usually light between 400-700nm, with anything under 400nm being Ultraviolet (UV) and anything over 700nm being Infrared (IR). In reality, you can see some light outside of this range however the light will appear very dim to human eyes. 400-700nm is also the wavelength in which most photosynthesis occurs.
White light typically encompasses 400-700nm with various peaks throughout this range, though different types of lighting may or may not output UV and IR light. For example, most white LED lights won't output very much UV or IR light while HPS typically will offer more in those ranges.
Here's an example of the light output from a CXB3590 LED from CREE
Infrared (IR): Infrared light refers to light above 700nm, though we do account for a little bit of light under this wavelength. 700nm light will look like a light red, and higher wavelengths looking invisible (think of a security camera with night vision, the light used is typically infrared). The reason why measuring IR is important is because it will affect leaf temperatures. The more IR a light produces the higher the leaf temperatures will be. Plants under HPS will usually have higher leaf temperatures because of IR light.
We currently show these two light parameters on the Air Sensor Lite as a percentage of the sensor's max. This reading should be used relatively. For example, if your normal light is registering 80% Visible light and you move the sensor to a different plant, you can try to get a gauge for how much light the other plant is receiving relative to the first plant. If the second plant is reading 40% then you'll know that it's only getting half as much light as the first plant is.
To be clear this is not a PAR (or PPFD) sensor, and thus cannot tell you if you have enough or too much light. A PAR sensor should be used to determine that, we recommend one of these options available from RapidLED for that purpose instead.