Definition
Colour temperature is the colour value emitted by a light source, expressed in Kelvins.
Introduction
This video by Phillips Lighting provides a good explanation of colour temperature:
As explained in the video:
When a solid body is heated, it emits visible radiation. The colour of this light is determined by the temperature of the object. This dependency is used to quantify the white tone of lamps. This so called "colour temperature" is used to objectively describe the white tone of a light source and is expressed in Kelvins, the standard unit of temperature.