In A Room of One's Own, Woolf critiques both patriarchy and essentialism. Patriarchy is a system of organizing society that puts a preponderance of money and power in the hands of males. Women are deliberately kept in a subordinate position. Essentialism is a theory that supports patriarchy by falsely asserting that essential, inborn differences between men and women make equality impossible.
Woolf wrote her essay to push back against the idea, still being voiced by men in the 1920s, that women's lack of achievement as writers was proof of their innate intellectual and creative inferiority. Woolf argues instead that women's underachievement is due to a lack of material resources. To do this, she describes being denied access to a university library solely on the basis of gender. She also describes the lavish beef and wine dinners the well-endowed men's colleges offer their undergraduates, while contrasting that to the poorly endowed women's college students subsisting on water and mutton. Men expect, too, to have rooms of their own in which to work, a luxury most often denied to women. By paying attention to such details, Woolf convincingly shows that women's lack of achievement is due to material circumstances, not essential inferiority.
Woolf uses a thought experiment to illustrate the way women are disadvantaged due to gender. She imagines a talented twin sister to Shakespeare, named Judith. Judith goes to London to be a playwright, but because of her gender ends up impregnated, devalued as a writer, and cast off.
It is patriarchy and essentialism that disadvantage women, not innate inferiority.