This is not a truly "fundamental" constant of nature like say the speed of light or the Planck constant.

Rather, it is just a definition of our Kelvin temperature scale, linking average microscopic energy to our macroscopic temperature scale.

For an ideal monatomic gas, say helium, there are 3 degrees of freedom. so each helium atom has average energy:

$3/2k_{B}T$

If we have 2 atoms at 1 K, they will have average energy $6/2k_{B}J$, and so on.

Another conclusion is that this defines temperature as being proportional to the total energy. E.g. if we had 1 helium atom at 2 K then we would have about $6/2k_{B}J$ energy, 3 K $9/2k_{B}J$ and so on.

This energy is of course just an average: some particles have more, and others less, following the Maxwell-Boltzmann distribution.