Light can have different color temperatures, indicated in units called Kelvin (K).

Bulbs with a low Kelvin value produce a warm yellow, more cozy light, while those with a higher Kelvin value produce a cool blue, more energizing light.

To see the difference, just move the slider.

As a reference, the color temperature of candlelight is about 2000K and bright sunlight is about 6000K. LED lights are between 2700 and 3000K, Halogen around 2800K and energy saving lights between 2700 and 6500K.