What follows is a very crude primer on feminism.
What is sex? What is gender?*
Feminists use the term gender to refer to masculine and/or feminine. Gender in this context refers to traits, such as intuitive, rational, valor, grace, etc. In any given era, a specific culture designates some of them as masculine and others as feminine. Feminists insist that the designation of some traits as masculine and others as feminine are a social construct. Our understandings of gender are historically and culturally contingent. Thus, being submissive was once an important component of being feminine but today one could be assertive and feminine. Feminists have suggested that we consider gender as a spectrum instead of an either/or binary. The most important insight from all of this is to recognize that all of us occupy shifting spots on this spectrum, depending on the context. Thus, I may exhibit more feminine traits in a private, intimate setting whereas the same I could exhibit traits associated with the masculine. In short, feminists would assert that gender is a social practice whereby human traits are arbitrarily designated as masculine or feminine.
Sex, on the other hand, designates whether we identify as male, female, both, neither. Sex is understood to be a binary, male/female. At the simplest level sex is about the physiological whereas gender is behavioral.
In most societies, the binary gender is mapped onto the binary sex. So, if we are born male we should display masculine traits, ie do not be feminine. Similarly, if we are born female we should display feminine traits. This could be summarized by paraphrasing Simone de Beauvoir, a French philosopher and a key 20th century intellectual: one is born female, one becomes woman.
At the most basic level, feminists attempt to disrupt the conflation of sex with gender. They assert that women can be masculine and men can be feminine or that we all have feminine and masculine aspects. Sex has little to do with gender.
A key concern here is that there are material consequences to being stereotyped as masculine or feminine. For instance most secretaries are women not because they have a special gene for note taking but because of how we define secretarial work. This tends to match our understanding of feminine traits. Needless to say secretaries earn significantly less than say someone designated as personal assistant, etc.
An equally important insight here is how the material world including the world of ideas tend to be gendered.
* please note that these definitions are very basic. The idea that sex is a binary has been scientifically disproved but it is a convenient way for me to summarize these ideas.