“People who are gender-fluid don’t identify with a fixed gender,” says Liz Powell, PsyD, an LGBTQ-friendly sex educator, coach, and psychologist. “They may move back and forth between gender presentations and identifications, or participate in queering of gender by mixing masculine and feminine presentations.”
The first thing to keep in mind is the difference between gender identity and gender presentation. As their gender identity fluctuates over time, gender-fluid people may ‘update’ their gender presentation to keep up with their identity and have more feminine and more masculine days. Or they may not change their presentation and instead choose to, as Powell says, “participate in queering of gender” by blurring the boundaries of masculine and feminine looks and exhibit their own version of what their gender presentation looks like. After all, our idea of what lies beyond masculine and feminine can’t be limited by the existing ways to present masculine or feminine.
On that note, some people get dysphoria on days where their identity doesn’t match the societal construct of what their body should be, some don’t. Dysphoria is defined as “a marked incongruence between one’s experienced/expressed gender and assigned gender,” according to the DSM-V.

Along with presentation, some gender-fluid people choose to update their pronouns (and even names) as their gender identity changes on a regular basis. It is important to be respectful of this––a simple way to be mindful is to just ask them what their pronouns are, and if they change based on how they present or if they would like you to check at regular periods.
The second thing to remember is that gender-fluid people may switch between genders every day, or identify with the same gender for a while––the fluidity of gender is individual and every gender-fluid person experiences this differently. While many gender-fluid people find their comfortable pattern, this isn’t something they control, and the periods of these shifts in identity can be disconcerting––like the problems mentioned in the tumblr screenshot above.
Going back to @naimitaj’s question––where do gender-fluid individuals fall? On the graph, they are a dynamic point moving freely between genders as their identity changes.