As the other responses indicate, Beauvoir’s quote argues that gender is a social construct rather than an inborn identity.

Through patriarchal structures and proscribed gender roles, female human beings are taught what it means to be a woman. This education is often achieved throughout childhood and young adulthood prior to a girl’s coming of age as dictated by the culture in which she is raised.

Interpreted another way, Beauvoir believes that traditional notions of femininity are not based on natural instincts but rather instilled from a young age. As a result, many women internalize these feminine tendencies as inherent, which further enforces male hegemony. This internalized sexism has just as much of an impact on a female’s identifying with traditional femininity as external forces, since it deepens an individual’s bias in favor of the status quo.

Simone de Beauvoir's comment "One is not born, but rather becomes, a woman" is a comment on the idea of femininity, womanhood, and maturity. She is saying in this quote that a person's sex does not make them into a woman—it makes them female. As they grow and mature, they will develop and their mind and thoughts, as well as their experiences, shape them into a woman over time.

This is a sad criticism of the world when taken in context with recent women's rights movements which highlight the cruelty and injustice suffered by women worldwide. In the light of those movements, it could be interpreted as saying there are many universal experiences that a girl must go through en route to becoming a woman, and most of them are painful.

These are the opening lines to Book II of The Second Sex, and what De Beauvoir is doing here is developing an important theme set out in Book I. There, De Beauvoir argued that femininity, as it is traditionally understood, does not arise naturally. It does not come about through differences in women's biology, psychology, or intellect. Femininity is an entirely artificial construct, arising not out of natural differences between the sexes, but out of their respective situations and the power they derive from them.

Men have traditionally held the power in most civilizations. Therefore, they have constructed an idea of femininity that keeps women in a state of permanent subordination. It is this notion of what femininity is and should be that women need to overcome and reject. In doing so, they will be asserting their true womanhood, one no longer subject to the shackles forged by unequal power and gender relations.