Gender expression is the outward signal of gender — clothes, hair, voice, body language, the way you take up space — and it's separate from the internal sense of who you are.
Think of it as two dials, not one. Identity is the private read — your own sense of your gender. Expression is the antenna: everything the room can see and hear. Usually the two dials point the same way, which is why people assume they're the same knob. They aren't. A man in eyeliner and heels is still a man. A woman with a buzzcut and a boiler suit is still a woman. Expression is never a verdict on identity — it's just what someone put on today.
Everyone has one, including the most straight, most cis person you know. A beard is expression. A harness is expression. So is a wedding ring, a power suit, a five o'clock shadow shaved sharp. Society files all of it under masculine, feminine, or androgynous — but that filing happens in the observer's head, not in the person standing there. On a dancefloor at 4am this stops being theory: expression is a language, the floor is fluent, and half the fun is watching people write in it.
The trap is treating expression as evidence. He dresses femme, so he must be gay; she's butch, so she must be a lesbian; they're androgynous, so they must be nonbinary. All wrong, or at best coincidence. Expression doesn't tell you someone's identity, their orientation, or who they want to go home with — it tells you what they chose to wear tonight and how they wanted to feel in it. Read the person, not the outfit.