Reification is a society-wide process where one person assumes something is real that isn’t. Like you say about nature, we think about “the market”. The market wants this, the market wants that, but it’s bigger than that.
To reify something is like to cast it into law, to make it permanent, almost like you’re taking ephemeral, concepty air and turning it into rock. It’s like to make something real. It’s important to note that when reification isn’t a solitary activity; so using a famous example from Althusser, when you’re walking down the street and you hear a police siren and a man yell “Hey you!”, when you turn around, you are reifying the policeman’s right to yell at you.
But how would I explain this to a small child with a concussion? I would say that the adult world is no more delusional than the children’s world. We still have invisible friends, we just call them commodities.