6500K LEDs are daylight white LEDs which have a bluish white light output. They are commonly used in applications which require lots of daylight, such as artist studios, manufacturing and other commercial environments. They are not often used in the home as the bluish tinge to the light can appear harsh, while yellower tones are more relaxing.
The ‘K' in 6500K stands for ‘Kelvin'. Kelvin is the Correlated Colour Temperature (CCT) which essentially describes the colour of the light output of the LED. The lower the number, the ‘warmer' the colour and the higher the number, the ‘cooler' the colour. 6500K sits at the higher end of the CCT range and has a daylight white appearance with a bluish tinge.