“good.” The Hebrew word is tov (#02896 טוֹב), the standard word for “good,” which has a huge semantic range. It is the word used in Genesis 1 when God said of His works that they were “good.” It refers to good of all kinds, including mental, moral, and material good. Thus it can mean things such as, “pleasant, agreeable; good, right, excellent; valuable, rich, prosperous; happy, glad; kind; appropriate; and better.” In this context, it clearly also includes “sound” or “accurate.” The best way to understand the teaching that the father gave is that it is “good” in lots of different ways. It would be sound and accurate, mentally and morally helpful, valuable, etc.
“instruction.” The Hebrew word is torah (#08451 תֹּרָה torah or תּוֹרָה towrah). Traditionally, torah is translated “law,” but that translation gives English readers the wrong impression (the Hebrew word mitzpah means “law” or “commandment”). Torah means much more than just “law.” On a basic level, it means guidance or instruction, but that instruction also includes doctrine, custom, theory, regulations, etc. “The Torah” instructs us through moral and legal regulations, examples, historical events, practical advice, customs, and the acts of God. We are to use Torah to develop sound thinking. A person who studies the Torah learns how to think like God thinks.
[For more on Torah, see commentary on Prov. 1:8.]