To begin with you need to understand what Fungible even means, fungibility is the property of an object to be exchanged or replaced by another one of the same class with both of them retaining the same properties, for example you can exchange one dollar bill for another dollar bill regardless of the conditions in which they are and they still have the same purchasing power, by the way most forms of money need to be fungible in order to work.

Following the same example there are one dollar bills that have some misprints and since each one of those mistakes is unique they have value outside their face value, which makes them more valuable but no longer fungible, non-fungible tokens fall into this category, each one is unique and their value depends on what people think each one is worth it, which is why there can be some NFTs which are worth millions while other similar ones are worth nothing.