why does the father have to explain what to do with the butter and biscuits?

This is when they find the underground bunker with all the stores of food and clothing. Some of the food, the boy had never experienced. It is implied that much of the boy’s life was lived post-apocalypse. Fresh butter and biscuits, as well as the can of Coke from earlier, were items that were hard to get during the boy's short life. This is why the father had to explain to the boy what to do.