Phenomenologists believe that there are individual instances or aspects (properties) of an object. For example, a white billiard ball is an instance of "whiteness." This instance of whiteness is different from an instance of whiteness of a second billiard ball. However, these individual aspects of whiteness are all of the same color and therefore suppose a universal whiteness or an essential whiteness.
Butler says that sex is biological and gender is a performative act. Gender is constructed or reified by performing certain acts such as wearing certain clothing, cultural interests, expressing sexuality and/or sexual preference, etc. Gender is performance. To be born with a female's anatomy is to be a female. But to be a "woman" is to engage in certain performative acts with stereotypes such as being interested in shopping, dresses, and so on. Since these are cultural constructed, they are arbitrary acts. In other words, these acts (performed as a gender) do not emerge from a universal or essential "woman-ness."
The essence of "woman-ness" is an illusion because gender is just performance. The notion that there is a universal woman-ness is, like the historical experiences of seeing whiteness, based upon repeated experiences of how a woman ought to appear and act. These notions have been repeated so much over time that they seem to be essential, immanent manifestations of some universal sense of woman. However, the repetition of such acts or roles has been to historical divide men and women into a convenient binary opposition.
The idea that gender is a performative act is based upon the phenomenological supposition that we intuit "woman-ness" from our experience of individual instances of "woman-ness." What Butler is saying is that there is no universal woman-ness. Therefore, gender is only the performative act. Since it is a socially and personally constructed way of acting, it is within human capability to change. So, clearly the old notions that women belong in the home, should not work, etc. are not based on some universal essence linked to biology or some abstract "womanhood." They are based on historical practices, some of which derive from a patriarchal domination of gender concepts.
A way to legitimately liberate women (and men for that matter) is to acknowledge that gender is a performance and that gender is not an expression of some abstract or universal notion.
In phenomenology, there are perceptual properties and abstracts. In Butler's analysis, the idea of gender is constructed out of acts and how others perceive those acts (properties); not, as some phenomenologists might think, out of some abstract universal. Gender (as male, female, transsexual, etc.) is therefore, a dynamic, changing concept.