The color of light, expressed in Kelvins (K). The sun is about 5800 Kelvins. This is called a "temperature," because the hotter fire gets, the more it changes from a warm yellow glow to white.
The lower the Kelvin rating, such as the common incandescent/tungsten bulb, the warmer, softer and more yellow the light. The higher the K number, the cooler, whiter and bluer the light.