Genderfluid describes a person whose gender identity isn't fixed: it shifts over time — sometimes across years, sometimes day to day — within the nonbinary spectrum.
Being genderfluid means your gender has a dial, not a switch. For some people it moves slowly, over months or years; for others it changes week to week, or with the room they walk into. On a given day you might feel like a man, a woman, both at once, or none of the above — and none of those readings is more "real" than the others. The movement isn't a malfunction. It's the identity.
The tired misreadings: that genderfluid people are confused, indecisive, or going through a phase they'll grow out of. Fluid isn't fickle — someone can be completely certain that their gender changes. It's also not the same as gender expression: swapping outfits or makeup is style, while being genderfluid is about who you are underneath, which is why pronouns can shift too (some keep one set, some rotate, some will just tell you). None of it needs your permission or your understanding to be valid.
Here's the part that annoys people who think this is a 2010s invention: the concept is ancient — cultures across the world have named genders that move for millennia. What's new is the English word, and even that is older than the hashtag. On a dancefloor the etiquette is refreshingly simple: read people as who they are tonight, not who they were last time, and when in doubt, ask. It costs nothing and it's the whole game.