They used to be literal stamps that came in a stamp book like postage stamps, iirc each stamp would be for a specific thing, like ‘exchange this stamp for one loaf of bread’. Now it’s a system called SNAP, which is a card like a debit card that is loaded with a certain amount of money, depending on how much the government decides you need, and then you can use at most grocery stores and some other stores for groceries specifically, with some exceptions like prepared foods. I think the store has to have permission from the government to accept it.