Social stratification is the separation between groups of individuals in society, and is typically determined by socioeconomic status, occupation, or power. In the US, stratification is seen in the differences between the lower, middle, and upper class.
In my day to day life, examples of social stratification are everywhere. Driving through downtown Seattle, one can see camps of homeless people in one area then drive 15 minutes north and be in a neighborhood of multimillion dollar homes. Social stratification is not a rare phenomenon, and is seen all throughout the world.
It is a system of categorizing people into socioeconomic strata based on different factors like power, occupation, ethnicity, or wealth.