A set of things is fungible if items are interchangeable: it doesn't matter which one of the set you own. Dollars are fungible, standard Snickers bars are fungible, the gas you put in your car is fungible.
Most things are not fungible. Real estate is non-fungible: you can easily distinguish one house from another (if only by its exact location on Earth). Da Vinci paintings are not fungible: there is only one of each. Loans are non-fungible: being owed money by a person you've never met is definitely not the same as being owed money by a close friend.
Some things are semi-fungible. Within all Yu-Gi-Oh cards made in the world, many different types of cards have been released (non-fungible), but there are several copies of each card (fungible). Or when you're buying a car, it might not matter for you personally which 2015 Volkswagen Golf you buy (fungible) but the cars are technically easy to distinguish using the VIN number (non-fungible).
Fungibility is not always objective. A collector might pay thousands of dollars for a rare $1 bill. Or you might be convinced that there's a world of difference between a 2015 VW Golf with seat heating and one without, and I might not care. That shows fungibility has a social component.