It's a vague label that means little more than that someone subscribes to some strain of body mod fetishism. It can be anything from "it is a moral imperative that we become literal vampires and microdose LSD and nootropics," to a desperate hope that medical science will solve all the problems of human frailty within the next few decades, to literal "the flesh is weak and abhorrent" 40K admech larping. Basically if you take the problematic and weird "ooooh nooo, they're defiling the sacred flesh with machinery! this will eat their soul and make them monsters!" shit from early cyberpunk and completely invert it, transhumanism is anything that could be reasonably described as that inversion.

A lot of prominent transhumanists are unhinged reactionary freaks with fantasies of becoming immortal ubermensch warlords, but IIRC there was also a big Soviet strain of transhumanism that came at it from the exact opposite angle and pushed the idea that socialist science could make everyone great no matter the circumstances of their birth.