There are of course a wide range of perspectives within feminist theory. The most widely discussed concepts of the feminist understanding of gender are as follows:
Gender in NOT a binary: This means that the traditional concept of gender (that there are only two genders: male and female) is entirely false. Not only is there a wide spectrum of genders between the so-called male and female ideals, there are also a great deal of genders which exist outside of this binary entirely.
Gender is separate from biological sex: This part somewhat depends on whether you are studying essentialist or non-essentialist feminism, but for the most part feminist theory states a person's gender is nothing more than a collection of behaviors. Theorists call these behaviors a gender performative. Gender behavior is mostly defined by societal pressures and personal choices, not biology.
Gender is fluid: Like all performances, no person will perform exactly the same behaviors for an entire life. Gender can change, both slowly over time, and at times even quickly to adapt to circumstances.
Gender can be a form of oppression: By forcing people into gender roles or discouraging people from performing their chosen gender, society forces both men and women into submissive roles. This reinforces institutionalized oppression within societies.
Gender theory is an entire field that goes well beyond literary studies. I hope this helps you get started.