Statistical mechanics is mechanics
applied to very large groups of particles.
It turns out that temperature
, for example, can be understood through mechanics
in this way! And the electromagnetic spectrum
of wavelengths that a body with a certain temperature
radiates can be derived by applying statistical mechanics to quantum mechanics
To say that "The temperature of this radiation is this" is to say that
a body with this temperature would radiate electromagnetic radiation with this
And the gas law PV/T = constant that you learn in elementary physics can be derived by statistical mechanics.
And you can use it to understand what heat capacity
means. And many other things..A good book about statistical mechanics (or physics) is Franz Mandl's "statistical physics". Some understanding of quantum mechanics is needed for some parts of statistical physics, but not all.