The property of fungibility is refered to the ability of getting replaced by an identical asset. It is the ability of the asset to maintain its standard value. The history of the currency should not be able to impact its value, and this comes from the fact that each piece which is a part of this currency should be equal in value when compared to other pieces belonging to the same currency.
In the crypto space it refers to the ability of the crypto asset to be replaced by another identical coin or token since they are interchangeable. Real world entities such as gold are also fungible as one kilgoram of gold has the same value as another kilogram of gold. However, when specific characteristics are added to the fungible good, say a serial number, it becomes unique and loses its fungibility, which means that it no longer remains equivalent in value. This is the case with non-fungible tokens or NFTs. A goodie signed by Eminem does not have the same value as one signed by another artist, say Travis Scott. These two goodies cannot be exchanged for the same value, and are therefore, called non-fungible.