What is kelvin?

The kelvin is just a special way to count heat that starts right at zero, so there are no negative numbers to confuse you.

Think about water. We usually say ice melts at 0 degrees Celsius, but in the kelvin scale, that point is actually 273. Why? because real zero, called absolute zero, is when atoms basically stop wiggling entirely. Imagine a busy playground of kids running around. At absolute zero, every kid sits perfectly still on their swing. No more bouncing. That is the true "nothing left" temperature.

Celsius starts at freezing water, but kelvin starts at absolute stillness. It makes math easier because if you have twice as many watts of power, you have exactly twice as much heat energy, not just "twice as hot" relative to some arbitrary point.

You use kelvin when talking about light bulbs or stars. A warm yellow bulb glows at around 2700 K, while a bright white computer screen might sit near 6500 K. It is just the standard scientific ruler for temperature, stripped of all the old fudge factors.