Thermal equilibrium
The best way to think about it is to imagine a huge
ensemble of systems that all have exactly the same
energy function.
The probability distribution is just the fraction of the
systems that are in each possible configuration.
We could start with all the systems in the same
configuration, or with an equal number of systems in
each possible configuration.
After running the systems stochastically in the right
way, we eventually reach a situation where the
number of systems in each configuration remains
constant even though any given system keeps
moving between configurations