Gender is often misunderstood and it's easy to see why people get confused about what gender really is, as we're often taught that gender is synonymous with sex. However, gender and sex are very different and complex.
Sex is the physical characterisation you are born with - terms such as AFAB (Assigned Female at Birth) and AMAB (Assigned Male at Birth) refer to this without implying gender. Gender is what you identify as, so you may identify with the gender you were assigned with at birth which would mean you are cisgender (cis being the Latin prefix meaning "on the same side of") or you could identify with the opposite gender which then would mean you are transgender.
Non-binary is an umbrella term for gender identities that aren't exclusively linked to male or female. People who feel like their gender is non-binary may feel as if they don't really have a gender, have more than one gender, or something else entirely.
Genderfluid is a non-binary gender and kind of means exactly what it says. For example, some days you may feel a lot more masculine, other days you may feel feminine and then sometimes you may feel as if you're kind of neither, however not everyone who is genderfluid feels this way.
There are lots of non-binary genders such as agender (not identifying with a gender), bigender (experience of having two possible genders which can be simultaneous or distinct), and pangender (identifying with multiple genders or all genders).
For more information on gender you can visit this page or genderspectrum.org.
Liah is an editor and regular writer for the StandOut MLC Newsletter, which this article was featured in. They are a member of the StandOut Committee and one of the founders of StandOut.