273 + degree Celsius = Kelvin. Why 273?

One Celsius (or Kelvin) degree as a temperature difference was defined as 1/100 of the temperature difference between the freezing point of water and boiling point of water.

We call these points 0 °C and 100 °C, respectively. The number 100 arose because we're used to numbers that are powers of ten because we use the base-ten system. The Celsius degree is "one percent" of the temperature difference.

There also exists the minimum temperature that one may achieve. In Kelvin degrees, it's sensible to call this minimum 0 K: that's a primary condition on the "natural" temperature scales such as the Kelvin scale. If one insists that one Kelvin degree is equally large as one Celsius degree as a unit of temperature difference, it just turns out that the melting point of ice has to be 273.15 K and the boiling point of water is 373.15 K. It's because the temperature difference between the boiling and freezing points is 2.7315 times smaller than the temperature difference between the minimum allowed temperature, the absolute zero, and the freezing point of water.

This number 2.7315 can't be explained in simple words. It is a fact about water, a fact of Nature that may be derived from the laws of quantum mechanics. One has to "simulate" what the water molecules are doing all over this large interval of temperatures and this is just what comes out. We know it is so from experiments.