Iron Fist is just a title
While Danny Rand is the Iron Fist, this superhero name is just a title. Just like Captain America was, for that matter, with quite a few superheroes taking up the indestructible shield when Steve Rogers wasn't around.
Danny Rand is just the 66th fighter to bear this title. And, as many would suspect, this title refers to the character's signature move – focusing chi energy into his fists. An Iron Fist can also manipulate the chi energy in order to heal, make himself more resistant, stronger, and so on.
The power of the Iron Fists comes from a legendary dragon called Shou-Lao the Undying, who is reborn within everyone who takes this title. So, in a way, the superhero Iron Fist is basically immortal.
Obviously, I don't expect to see any dragons on Netflix (the origin story of Danny Rand as a superhero involves him plunging his fists into the heart of the dragon). But I would very much like to see one!