Found a good explanation:
Colour temperature is a measure of the colour of 'white' as displayed by the
monitor. The value is expressed as the temperature of a 'white-hot' object
whose colour output matches the displayed colour. Higher colour temperatures
e.g. 9300° are more 'blue-white' lower temperatures e.g. 5000° contain more
red. Perversely, high colour temperatures are often described as 'cooler'
whites because of the blue component, whereas the redder whites at the
cooler colour temperatures give a 'warm white' appearance!