The K is the color temperature. What it means is, you would have to heat any object (it doesn't matter what, but very few objects can get this hot without burning up) to a temperature of 4300 kelvins (subtract 273 to get celsius) to get it to glow the same color as these bulbs. That said, light from hot objects (like lightbulb filaments) is full-spectrum and is more pleasing to the eye, whereas HID, LED, and fluorescent lights are not full-spectrum. They only have a couple of colors of light in the mix, whereas traditional bulbs have ALL colors of light. That's why traditional bulbs are better for seeing, though it's difficult to get them as bright as HIDs can be.Do you know what the K is? (I dunno how to ask that question). What's the temp/brightness/blah.