Colour is expressed as a temperature due to the relationship between the colour of light and temperatures when objects are heated to high temperatures. There is a longer explanation to this but for adjusting colour temperatures on LCD monitors, we don't need to understand it.
As mentioned, colour temperature refers to the colour of light and this serves as the standard index for colour balance for a range of products, including monitors, cameras, and lighting equipment. Colour temperature is specified in units of Kelvin (K) of absolute temperature, not the degrees Celsius (C) used to express the temperature of air and other materials.
Basically, the lower the Kelvin value for colour temperature, the redder a white object appears; the higher the color temperature, the bluer it appears.
Most photographers shooting pictures with DSLR cameras might set their white balance to 5000-5500 K. This is because daylight has a colour temperature of 5000-5500 K and setting the white balance to this figure makes it possible to capture photos with colour reproduction close to that perceived by the eye.
Here is a diagram of colour temperature. Please note that this diagram is merely a rough representation of how to think about colour temperatures, not a precise indication of colour temperatures under specific conditions.
You can learn more about colour temperature on an LCD monitor here.