What is Kelvin a measurement of, and how do you use it practically?
Kelvin is used to measure the color temperature of light sources, where lights which output below 4000 Kelvin have more of a red/warm light output, and lights above 4000 Kelvin and around 6500 or higher appear blue/cool.
When looking for LED lights, for instance, the Kelvin measurement is always present in the product specifications; if it's a daylight LED light the Kelvin value will be around 6000 or 6500 K, and if it's a tungsten LED light the Kelvin value will be around 3000 - 3200 K.
Different light colors are good for different things. Daylight lights work best at simulating daylight, where tungsten simulates interior lights, sunsets or specific skin tones the best.
History and the name "Kelvin"
The name "Kelvin" comes from the physicist and engineer William Thomson, 1 st Baron Kelvin (born 1824, dead 1907) who the base unit is named after.