Social stratification can be defined as the situation in which a society is divided in a hierarchical way. In a society that is socially stratified, groups are arranged in a hierarchy with each level (each stratum of society) having different degrees of power and wealth in the society.
For example, our society is generally stratified largely on the basis of wealth. Those who have wealth generally have power and are part of the highest level of society. Those who have very little wealth have little power and are at the lowest rung of society.
In other societies and other times, there has been social stratification, for example, on the basis of birth. In such societies, people who are born to aristocratic families occupy the highest levels of society regardless of how rich they are. Those not born to such families cannot achieve the highest level regardless of their wealth.
Social stratification, then, occurs when society is divided up into different levels with different amounts of wealth and power.