Statistical thermodynamics is the study of how the tiny particles in matter (like atoms and molecules) give rise to the large-scale properties we can measure — such as temperature, pressure, and energy.
It uses statistics to connect what happens on the microscopic level (particles and their energies) to what we see on the macroscopic level (like gas expanding or a solid melting).
In simple terms:
It explains why the laws of thermodynamics work, by looking at how countless particles behave together.