SES is short for socioeconomic status, a concept used in a lot of social sciences research to identify different levels of material wealth, social influence, and group power in a given population. Socioeconomic status categorization is based on things like how much money you make, what part of town you live in, whether you have substantial savings, and whether you belong to a race or ethnicity which is treated differently from the norm in your society. In many ways it's a replacement for the less precise concept of class.

More information at socioeconomic status. The abbreviation merits its own node because in most scientific contexts it is rarely or never expanded.