Temperature is proportional to the speed of the molecules moving around in a substance. It is therefore proportional to the average kinetic energy of those molecules.

The Kelvin scale is also known as the absolute temperature scale because there are no temperatures below 0, which is the point at which all molecular and atomic motion stops (the speed of the motion is proportional to temperature on any scale). Physicists choose the Kelvin scale for thermodynamic equations so that they don't run into the problem of negative energy, which may result from the use of negative temperature values.

Note that there is a constant difference of 273 between Celsius and Kelvin temperatures. This means that the Celsius "degree" is the same increment of temperature as a "Kelvin" (there are no "degrees" in the Kelvin scale; the actual temperature is the Kelvin).