On the entry of Gibb's entropy formula on Wikipedia, the following definition is given: "The macroscopic state of the system is defined by a distribution on the microstates that are accessible to a system in the course of its thermal fluctuations." (I will assume that this description is correct. If not, this question does not apply.)
I'm having a hard time grasping exactly what this description tries to say. I initially thought that the distribution of microstates was given by which microstates that could instantiate the macrostate in question. However, both "accessible" and "in the course of its thermal fluctuations" makes me wonder about if this interpretation is correct.
When reading the description above, it seems to me that you should start of with a certain microstate that instantiate the macrostate and then see how many states that are accessible, that is, that could be the case within the time course of some time unit defined by the thermal fluctuation...Ehh....I might have lost myself there.
Could somebody deconstruct the description cited above?