# Precise meaning of Fundamental assumption of statistical mechanics

Introduction to Thermal Physics by Schroeder explains the fundamental assumption of statistical mechanics as,

Let's assume that, over long time scales,the energy in a system gets passed around randomly in such a way that all microstates are equally probable. So if you look at the system at any instant, you are equally likely to find it in any of the possible microstates.

While I understand the big picture of what this assumption is saying, I have questions about the precise details. Why is it important that we consider long time scales? And what does it mean by "equally likely"? Is it saying that if we looked at the system, say every 1 second, for the next 100 hours, each and every microstate will occur with the same frequency?