A supervisor is the person in charge at your job, who monitors you as you work. Your supervisor prepared you for the world of sneaker sales, and makes sure you don't give them away free to your friends.
To supervise means to oversee, so a supervisor is someone whose job it is to oversee everyone else as they work. If you're the supervisor of a big kitchen, you probably won't spend time chopping onions, but instead wander around making sure everyone has their hair back and is using the right ingredients and working quickly. Sometimes in very complex problems, a whole organization can be a supervisor, like a city department that oversees the police.