Appreciating fungibility
Fungibility is the property of perfect substitutability: one mint condition five euro note has the exact same value as any other comparable condition five euro note. One minute of my time now may be identical in value to one minute of my time 30 seconds from now.
Fungibility is desireable in the sense that it provides optionality. Having multiple modes or means to achieve a particular end, without being bound to a single approach, is self-evidently useful. Being able to identify things that are fungible for one another quickly likewise provides strategic advantage.
Identifying infungibility
The derivatives of fungible things are very often infungible. People will commonly mess up decisions under the false delusion that something is fungible, when in fact it is at least in part not.
For example, while time is strictly fungible, timing is not. We can swap 5 minutes now for 5 minutes later, but the rest of the world continues to move on, and what you can do in those 5 minutes might not mean the same thing, and the options available to you later might differ. See also: speed.