Essentialism is the doctrine that ascribes a fixed property or 'essence' as universal to a particular category of people. To propose that women are good childcarers because they are women, that black people are good at sports because they are black, that jews are good at arguing because they are jews - all because 'they are like that' - is to engage in essentialist thinking. The basic principles of stereotyping any cultural grouping operates on essentialist lines.
In cultural studies there is a wariness of essentialist reasoning for at least four reasons1 positing supposedly' essential' characteristics often simply involves the reproduction of judgements of one group about another _ essentialist reasoning imposes a partial set of judgements grounded in the situation and interests of one social group upon another
2 essentialist accounts of persons and activities as typical usually involve enormous over generalisation which ignores the differences between the members of a category ( are all women really like that? all black people )
3 it follows that essentialist reasoning cannot explain why these differences within a category exist in the first place