Light has different color tones at different times of day, under different weather conditions, and when generated by different types of artificial light sources. On a late afternoon the light might have a warm reddish tone only to change to a bluer tone as dusk sets in. The light from an incandescent light bulb is warmer (redder) than that from a florescent light tube.
The differences in the tones of light in photography are measures in Klevin degrees. I am not going to try to explain the scientific significance of Kelvin color temperature. I have neither knowledge or space to do that here. If you want more information about Kelvin just Google it. You will find a lot to read. In photography the Kelvin scale ranges from red (warm) to white (neutral) and then to blue (cool). Kelvin is measured in degrees with 2500 K being very cool and10,000 K being very warm. We call those degrees color temperature.
Digital photo processing allows us to change the tonality of light. Since the tone of the light can affect the mood of a photograph by adjusting the color temperature in post capture image processing we can substantially alter the look of a photograph. This process is known a adjusting the white balance. Most digital image processing software has a white balance control setting. It might be called white balance, color temperature, or temperature.
The accompanying gallery has some examples of photographs that have been process to different color temperatures. Take a look and you will see how the adjustment can affect mood. In each series the first photo is as shot in camera with the camera set to auto-white balance. The second photo is cooler, and the third is warer