Essentialism is traditionally the view that every entity has a set of attributes that are necessary to its identity and function. Essentialism in this sense originates from Plato and his theory on the "essence" of things.
See the article on race and in particular the section "Definitions of race" regarding the above being one possible way to define races. However, it is not the definition used by scholarly race realists who have used more "fuzzy" definitions as discussed in the article.
In a rather different sense, "essentialism" is also a concept in various "post-modernist" and Cultural Marxist theories. It is in these theories a rather unclear concept that is disliked and criticized, and should possibly be seen as actually referring to disliked and genetically influenced group differences and in particular to disliked and genetically influenced race and sex differences.
"Essentialism" in this sense may involve straw men arguments regarding the role of genetics, such that genetic proponents are falsely claimed to state that genetic factors determine everything, when they instead typically argue that both genetic and environmental factors are important.