What does this aphorism mean: "A good example is the best sermon."?
It means that doing something good can be inspiring to another who sees it.
An aphorism is a saying that carries deeper meaning.
In the above aphorism, the word sermon refers to a religious or moral talk that is meant to give inspiration on good behaviour, responsibility, concern, etc.
When one does something good, it has the potential to set an example for others even more powerfully since those seeing the good example are more likely to be inspired to imitate that example.