The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of a color comparable to that of the light source.
When the ideal black-body radiator is heated, the color of light it emits will change. This color begins as red in appearance and graduates to orange, yellow, white, and then blue-white to deeper colors of blue. The temperature of this metal is a physical measure in degrees Kelvin or absolute temperature. While lamps other than incandescent such as LEDs do not exactly mimic the output of this piece of ideal black-body radiator, we utilize the correlated color temperature (or Kelvins) to describe the appearance of that light source as it relates to the appearance of a black body radiator.
Light is often referred to having a certain color temperature. Strangely, warm light is a lower color temp, and cool light, blue, is a higher color temp.
Here are some temps of some common light sources: