Kelvin is a scale used to define the colour temperature of a light source; abbreviated as “K”. (videomaker.com)

Have you noticed your photographs have a blue hue to them when photographing in the shade or a red/orange hue when photographing inside near tungsten lighting?  This is caused by the Colour Temperature.  Higher colour temperatures (5000K or more) are considered to be cool colours (bluish white) and lower colour temperatures (2700-3000K) are thought to be warm tones (yellowish white to red).

To achieve better colour balance in your images, you can manually set your Kelvin scale.  Here is an excellent chart to help you with your colour balance.  I always carry a copy of the Kelvin Scale in my camera bag (click on the scale to print).