It is measured in degrees Kelvin.
Light sources produce a mixture of hues.
As the light source becomes bluer in color, its color temperature will increase and as it becomes redder, its temperature decreases. The reason that color temperature is important is that it will affect the color of the subject lit by the light source. Here is an example better explaining the effects of color temperature. Let's say you are shopping at a department store and spot a garment that looks appealing in its color. Noting the color, you purchase the garment and go home.