What does Psalm 37:31 mean?

In this verse David tells us why the mouth of the righteous utters wisdom (Psalm 37:30). Those who are truly righteous have the Word of God in their heart. Jesus said that "out of the abundance of the heart the mouth speaks" (Matthew 12:34). If we give God's Word a heartfelt welcome, it will produce wisdom. What we say and what we do will be controlled by wisdom (Psalm 119:9–16, 104, 129–130).

When God commissioned Joshua to lead the Hebrews into Canaan, He told him to meditate on the Book of the Law day and night and to "be careful to do according to all that is written in it" (Joshua 1:8). He promised Joshua: "For then you will make your way prosperous, and then you will have good success."

Psalm 1:2–3 employs the analogy of a fruitful tree to describe the person who delights in God's Word and meditates on it day and night. Love for the Word makes a person like a well-watered tree "that yields its fruit in its season, and its leaf does not wither. In all that he does, he prospers."