Kelvin is the scale in which the color temperature of light is measured.
Warm light (candle light or sodium lights) are at the lowest end of the scale, coming in around 1700 Kelvin. Clear blue sky comes in at the highest end of the scale going up to roughly 27,000 Kelvin.
Interesting, but why does this matter to photographers?
It matters because of the white balance feature in your camera. Except at the farthest ends of the scale, our eyes perceive white as white regardless of the color of light illuminating it. Our brain interprets the light in relation to the colors around it. Cameras on the other hand can't do the intelligent interpretation our brains do to interpret the light temperature and adjust the other colors to fit within the range of whatever it deems to be white, which can color the picture in a way you don't intend. Also, when you mix lights from different sources with different values on the scale, you can confuse the camera entirely and cause color to shift in a way that doesn't reflect reality. Sometimes that's a nice effect, sometimes it's not. As photographers, we need to be aware of the color of the light source and either use them or compensate for them in our photographs.