I've only seen the film The Road, I haven't read the books. Perhaps I missed it, but what was going on with the people missing their thumbs?
According to the director (who's paraphrasing the author of the book) it's "open to interpretation, I think some kind of primitive punishment..."
You can find the answer in the book, when the man and the boy went after the man who stole their cart:
Get away from the cart, the man said.
He looked at them. He looked at the boy. He was an outcast from one of the communes and the fingers of his right hand had been cut away. He tried to hide it behind him. A sort of fleshy spatula. The cart was piled high. He’d taken everything.
Get away from the cart and put down the knife.
~ Cormac McCarthy’s The Road, page 273 (Picador, London, 2007)
It was a punishment for leaving (or being kicked out of) a commune.
There's a wonderful thread discussing the different approaches and theories regarding the missing thumbs here.