In her most influential book Gender Trouble (1990), Butler argued that feminism had made a mistake by trying to assert that 'women' were a group with common characteristics and interests. That approach, Butler said, performed 'an unwitting regulation and reification of gender relations' -- reinforcing a binary view of gender relations in which human beings are divided into two clear-cut groups, women and men. Rather than opening up possibilities for a person to form and choose their own individual identity, therefore, feminism had closed the options down.
Butler notes that feminists rejected the idea that biology is destiny, but then developed an account of patriarchal culture which assumed that masculine and feminine genders would inevitably be built, by culture, upon 'male' and 'female' bodies, making the same destiny just as inescapable. That argument allows no room for choice, difference or resistance.
Butler argues that we all put on a gender performance, whether traditional or not, anyway, and so it is not a question of whether to do a gender performance, but what form that performance will take. By choosing to be different about it, we might work to change gender norms and the binary understanding of masculinity and femininity.
This idea of identity as free-floating, as not connected to an 'essence', but instead a performance, is one of the key ideas in queer theory. Seen in this way, our identities, gendered and otherwise, do not express some authentic inner "core" self but are the dramatic effect (rather than the cause) of our performances.