Macroscopic states hide enormous amounts of microscopic detail. A gas specified only by pressure, volume and temperature can correspond to many different arrangements of its molecules.

A microstate specifies microscopic detail; a macrostate specifies only the coarse quantities being observed.

Multiplicity and entropy

If a macrostate can be realized by $\Omega$ compatible microstates, Boltzmann's relation is

$$S=k_B\ln\Omega.$$

Macrostates with larger multiplicity are overwhelmingly more probable because vastly more microscopic configurations realize them.

Why equilibrium is typical

A gas initially concentrated in one part of a container can spread throughout it. The reverse microscopic motions are not forbidden by classical mechanics, but the dispersed macrostate corresponds to enormously more microstates than the concentrated one.

The system therefore almost always evolves toward macrostates of higher multiplicity.

The thermodynamic arrow

The microscopic dynamical laws can be approximately time-reversal symmetric while macroscopic processes exhibit a preferred temporal direction. This arrow of time arises statistically from low-entropy initial conditions and the overwhelming typicality of higher-entropy macrostates.

The second law is therefore not a new force pushing systems toward disorder; it is a statistical statement about what macroscopic behaviour is overwhelmingly likely.