cold iron just means iron, or more precisely iron not hot enough for forging. That said this is really up to you and how magic/supernatural things work in your universe. as one author put it the answers range from subatomic resonance to becasue god said so. If you don't want this to be closed or being too broad you need to give criteria for how to differentiate a good from a bad answer.

@John I'll try and think of some criteria

Cold iron is wrought iron, i.e., iron that can be worked by hammering without heating, as opposed to cast iron, which is poured into casts or moulds when molten.

Possible duplicate of Plausible pseudo-scientific explanation for cold iron?

@nzaman that's not what wrought iron means.

@John I don't personally think it's a duplicate because that question is about making cold iron. Mine's about why it harms faeries

@John -- Also, that query specifically addresses ferromagnetism while this question is more general.

OP: Please be very careful when editing to add stipulations after answers have already been given!! You risk invalidating answers and are inviting your respondents to revert your query back to its original form.

@elemtilas I understand. I didn't want this question closed so I edited to try and fit the two answers I had already.

@John enworld.org/threads/cold-wrought-iron-what-is-it.157271

@CelestialDragonEmperor since we don't know what makes your fairies work, any answer we give will be pure opinion. since the magic system is up to you why iron bothers fairies is also entirely up to you.

I wonder if the VTCers could explain their rationale. I read this as a basic "how the world functions" query. It could indeed benefit from some work, per John's comment about how Faeries work, but other than that I fail to understand why this was closed.

@elemtilas same here. I'm a bit confused why this was closed. At first I didn't have criteria for the answer but I added that.

@John I didn't want to add how the fearies work because I thought to much backstory would lead to it being labeled off topic

@CelestialDragonEmperor how faries work is basically the only thing we need to know to say why iron would bother them.

@John I added how faeries work. Do I need to add a bit more? (Sorry for so many comments)

@CelestialDragonEmperor I think you best bet is to come up with a reason you link and ask if anyone can find any problems with it. I doubt your question will get reopened. justa thought obligations, deals, names these are things of order maybe the spirits are creatures of chaos so weak to order they can force order on them, iron as the most stable element (specifically iron 56) also contains enough order to permanently alter them, whether by chain or blade.

@CelestialDragonEmperor I started a discussion about questions like this one in the meta chat: worldbuilding.meta.stackexchange.com/questions/7628/…