I'm just trying to understand how a person can buy a fish and not know what kind it was

Pennsylvania, 1956. Frank Sheeran, a war veteran of Irish origin who works as a truck driver, accidentally meets mobster Russell Bufalino. Once Frank becomes his trusted man, Bufalino sends him to Chicago with the task of helping Jimmy Hoffa, a powerful union leader related to organized crime, with whom Frank will maintain a close friendship for nearly twenty years.