Possible meanings are 1) "If you make a tree good, its fruit will be good, and if you make the tree bad, its fruit will be bad" or 2) "If you consider a tree to be good, it will be because its fruit is good, and if you consider it tree to be bad, it will be because its fruit is bad." This was a proverb. People were to apply its truth to how they can know whether a person is good or bad.
"Fruit" here is a metaphor for what a person does. This can be stated in active form. AT: "people know whether a tree is good or bad by looking at its fruit" or "people know whether a person is good or bad by looking at the results of that person's activities" (See: [[rc://en/ta/man/translate/figs-activepassive]] and [[rc://en/ta/man/translate/figs-metaphor]])