This saying is a clearly metaphor to explain how, for many different reasons, other people appear to be always in a better situation compared to us, although they might not be. This feeling can be caused by many different things: other people can appear happier, fulfilled, richer and so on…but in the end what we see doesn’t always reflect how things really are.
This saying reminds us that most of the time we don’t give the right value to what we have and we are always admiring other people’s lives or what they own, forgetting to appreciate our own “fence”.