It's treating commodities as things, not as relations, and forgetting that humans make commodities.

For example, it's easy to look at a book and see a block of paper with writing on each sheet, all glued together. If it's a signed book, it might fetch a higher price than one that isn't signed. If it's brand new, it might fetch a higher price than a second hand copy. The commodity – here, the book – is treated as a thing. This isn't 'wrong'; but it's one-sided.

At the same time, the book is all the people who put it together: the writer, the printer, the binder. And all these can only be achieved if other workers have built a building, a printer, a laptop/typewriter, roads and vehicles to transport the equipment, materials, and the final product. Every one of the workers in these processes had to eat food every day (grown by farmers, prepared, packaged, cooked, etc), had to be raised and educated, etc. In reality, then, the book is a combination of all the relations between all the people who were needed to turn an idea into a book and get that book into shops and to each reader.

When commodities like books are fetishized, they're treated as things and all the human relations are ignored. This was hard to do before modern capitalism, because many fewer people were involved in creating goods. As you knew who was involved in making e.g. a book (a small guild of craftspersons), it was harder to forget that the only reason the book existed was because of labour.

Under capitalism, labour is divided into so many parts, that very few people in the production process know anyone else. One person chops trees with a chainsaw. Another turns trees into pulp with a machine. Another turns pulp into sheets with another machine. Another cuts sheets into pages with a guillotine. Another makes chainsaws. Another gears, oil, chains. Another machines. Another guillotines. Another turns chemicals into ink. Etc, etc.

None of these people know each other personally. They only know each other as the commodities they produce.

When we go to the shop to buy paper, a book, ink, or anything else, we 'meet' all the people who have produced those goods but we only see the commodity, not those people. We come to fetishize commodities, hold them up as things that appear in shop shelves as if by magic.

If we want to solve a problem, we look for commodities as the solution. Hungry? Buy a sandwich. Thirsty? Buy a drink. Fancy a story? Buy a book. Cold? Buy a coat or a house or a heater. We are working together to meet each others needs but we do it by buying commodities. So the commodities take on some magical properties.

As money is used to buy any commodity, we eventually come to fetishize (worship) money as having unique magical properties. With money, one can buy anything they want or need. It can fix every problem. Or so it seems when commodities are fetishized. But it's human labour, human relations – society – that produces the goods (and services) that satisfy our needs and wants, that solves our problems.

Commodity fetishism turns relations into things and separates humans from one another by making it appear that we don't need each other to survive. This has loads of distorting and disturbing effects by encouraging us to be anti-social.

It's human labour that produces what we need to survive. When we fetishize commodities, we come to think that it's the commodities that keep us alive, not the human labour that produced those commodities.