The Boltzmann constant (k or kB) is a physical constant relating a gas's mean particle kinetic energy to the gas's temperature. For particles with no energy tied up in rotation or oscillation, i.e., single atoms:
mean ke = (3/2)kT
The Boltzmann constant is the gas constant divided by the Avogadro constant. Its value is 1.380649 × 10-16 erg/K. It is key in statistical mechanics, the relating of statistical properties of many individual particles to physical characteristics we perceive at our own scale.
The Kelvin scale was re-standardized in 2017 such that this value of the Boltzmann constant is exact. Given the above formula, any temperature corresponds to an amount of energy and vice versa, even though in daily life we perceive them as something different. The new standard was considered reasonable because careful measures of the Boltzmann constant under the previous Kelvin scale standard were producing the same value repeatedly and reliably.