What is a clear a concise conception of Marx's notion of commodity fetishism?

The fetish in religion is an object that is confused with the spiritual forces it merely represents. Thus a crucifix or carved bone or chant, the thing itself, is thought to hold healing powers or other immaterial forces. The iconoclasm of Islam and early Protestantism was an effort to dispel such common human tendencies.

For Marx, the counterpart of "spiritual forces" is the complex of social and material relations by which a society produces and reproduces itself. The commodity is an object produced by labor for exchange and thus with an exchange "value" in addition to its "use value." Yet for us consumers this object appears simply to have a "value" in itself. This may be real enough, not an illusion. But it tends to conceal and misplace the vast global complex of labor, law, and other social relations that produced that iPhone and through which its "value" originates.

Marx does not define or discuss commodity "fetishism" at any great length. Yet it is a highly fruitful concept. The critical thing is to grasp the centrality of the commodity for Marx, with which he brilliantly opens Das Capital, as if it were the vital clue in a detective story.

That nearly everything produced is produced for exchange is the defining feature of capitalism. The commodity is the essential unit and very being of capitalist society. Everything is both what it materially is (its use value) and something else (its exchange value). This includes, of course, the commodity of labor, which is also "produced" for exchange.

So, can the labor commodity also be fetishized? Indeed, it often is. When the Board pays the CEO a thousand times as much as the average worker for his/her "wealth-generating" powers, that slice of labor commodity is being fearfully and superstitiously fetishized in the hopes of appeasing the gods of risk.

One very broad consequence of this is that, for Marxists, you are not going to fix social problems simply by conjuring with exchange values--for example, raising wages. This may alter prevailing beliefs and prices, but it does not indicate a real change in the social relations and underlying use values constituting social reproduction.

Marx uses the notion of a fetish to point to a kind of magical thinking people have with respect to commodities. Adopting your example, suppose that Mary feels cold and wants to do something about that. She has a choice:

• Solve the issue herself: e.g., knit a sweater, build a fire, etc.
• Convince someone else to solve the issue for her: e.g., have them knit a sweater, have them build a fire, etc.

As the saying goes, sweaters do not grow on trees; someone has to knit that sweater, somehow and somewhere. Even if Mary knits it herself someone has to teach her how to do so, somehow and somewhere. Acquiring a sweater (or any other item that solves the issue of 'coldness') is an inevitably and irrevocably social behavior. The world is a buzzing web of social interconnections, with people making things that other people can use, so that those other people can make other things to be used by still others.

However, the notion of commodities hides the 'social' nature of the interaction. When I order a sweater from Amazon, it comes from a warehouse in a cardboard box. I never see the people who knitted it; I never see the Amazon employees who boxed it up and shipped it out. As far as I know that sweater did grow on a tree in some special hydroponics vat run by Amazon-designed robots. That is the fetishism Marx is talking about: the strange way in which commodities seem to simply exist without any origin point. The human social interaction of making, selling, and buying has been effaced and replaced by the purely abstract economic notion of an interaction between commodities and money, to which people are (at best) tangentially related.

This commodity fetish plays into the Lockean fantasy of robust, completely self-sufficient and independent individuals: people who acquire everything they have through their own solipsistic efforts. The exchange of money and commodities masks the fundamental fact that both money and commodities are generated by the social efforts and interactions of human beings. Poverty, starvation, inequality, classism, and other social ills can be shrugged off because no one sees the connection between their purchase of a commodity and the labor that others invested in creating that commodity. One can see Michael Jordan pitching Nike shoes and never think about the fact that Jordan was paid more for his branding than the collective wages of all of the south-east Asian workers who physically made the shoes. It seems as though Jordan waved his hand and Nike suddenly discovered a trove of shoes with Jordan's brand name, and is generously sharing their discovery with us; or as though the shoes were somehow a natural, inevitable byproduct of Jordan's basketball career.

It would have been more ironic if I could have used Magic Johnson as an example, but I have to work with what the world gives me... But really, that's what Marx is talking about: this 'magical' appearance of commodities in the economic framework, lacking all connection to the inherently social processes that brought them into existence.

if Mary says that insulation is a benefit of (wearing) a sweater, then she fetishizes the sweater.

Do I fetishize the metal lead by ascribing malleability to it?

Not quite. These kinds of material facts relate to Marx's conception of use-value, which is not in any way particular to capitalist production, and is quite distinct from his concept of value, which is unique to capitalist social relations. The insulating property of the sweater or the malleability of a metal would be the same even if these objects were not produced as commodities. So they really don't have much to do with commodity fetishism.

The primary object of Marx's critique of commodity fetishism was the discipline of political economy as it was practiced in his day. The same critique could apply to modern economics, for whom prices are determined simply by supply and demand, without regard to relations of production or the special value-producing quality of human labor. In other words, for Marxists, mainstream economists confuse exchange-value with value itself. You are correct that Marx saw this as a "supernatural" kind of thinking, attributing properties to commodities themselves which are in fact the product of social relations and human labor.

Above all, I would emphasize that the concept of commodity fetishism is about a misunderstanding of value, and that more specifically value is a uniquely capitalist phenomenon quite distinct from use-value. So it's not really about causal chains either, but rather how the concept of value is defined and understood.

For further reading I would recommend various brief explanations for starters like this one from Oxford Reference or this one from a course at Purdue. The original source is Marx's chapter "The Fetishism of Commodities and the Secret Thereof".