Henslin defines social stratification as "a system in which groups of people are divided into layers according to their relative power, property, and prestige" (2009, p. 172).
The whole notion of stratification is borrowed from geology where layers of sediment, organic matter or stone, called strata, are evident when one digs down into the ground from the surface.
Sociology borrowed that term and made the analogy that in a given society, some people (the poorest or least powerful) are at the bottom of the pile. Others with more wealth or power are over them, still others who have even more wealth or power are on top of them, until the wealthiest or most powerful are at the top of the heap.
No known society is or has been unstratified! Someone always has more money, goods, power, than someone else!