An essentialist idea is one in which certain types of people are said to have inherent characteristics that are innate to members of that type of person. Therefore, any social science idea that holds this to be true would be called essentialist. The clearest examples of such ideas are some strands of feminism and of racial/ethnic studies.
In these two types of social science ideas, there are said to be traits that are inherently female or inherently (for example) African American. These traits are typically said to be good ones. For example, feminist theorists of international relations sometimes argue that having more women in positions of power would reduce the amount of conflict in the world because women are less confrontational than men. Some extreme "Afrocentric" views of the world argue that people of African descent are less materialistic and more spiritual than white people are. Such ideas would be called essentialist by postmodernists because they assume that certain groups have inherent characteristics.