In physics a black body is a theoretical object without reflective properties. When it is heated it emits visible wavelengths dependant on temperature. It glows red, orange, yellow, white and finally blue with increased temperature. Stated in Kelvin, the "Colour Temperature" is the absolute temperature required for the black body to radiate a certain colour.
Kelvin is an absolute measure of temperature. A single Kelvin (K) (not expressed with a degree symbol) has the same interval as degree Celsius, however 0K = -273.25°C.
Our eyes automatically compensate for colour temperature. A page of a book can look equally white in daylight at midday and in tungsten light in the evening. However when recording an image on camera without adjustment, the same page can look blue in daylight or yellow in tungsten light. In order to get an accurate rendition of what you experience in person, it is critical to adjust for the colour temperature of the ambient light. When using film the adjustment is made by selecting the correct physical media.
In practical terms cameras are set to either Daylight/Outside (approx. 5600K) or Tungsten/Inside (approx. 3200K). In order for white objects to appear white it is important to light the objects with the appropriate lighting source. It is possible to adjust the colour temperature of a lighting source by the use of filters. A blue filter can be used to adjust a tungsten source to produce daylight. And a yellow filter can be used to adjust a daylight source to produce tungsten light. A wide range of colour correction filters exist to balance most common light sources to appear the same on camera.