We saw that the person, whose delight is in the law of the Lord and meditates on it day and night, is like a tree planted by the streams of water. It is planted (secure) drinks deeply from the water (sustainable); it yields fruit in its season (timely); its leaf does note whither (enduring). The summary statement of this person is found at the end of verse three—in all that he does, he prospers. The one who is directed by the wisdom of God, that is found in the word of God, is rooted, fruitful and prosperous.
This person is contrasted with those who find their direction elsewhere. In contrast to the fruitful, enduring and prosperous outlook, they are like chaff that the wind moves out of place and it is forgotten.
On the cross, the man Jesus, delighted in the law of the Lord, fulfilling it through his life, death and resurrection. In trusting Jesus, his death becomes the payment for our failure to delight in the law. In union with the Jesus who died on a tree, his righteousness is gifted to us and we can now become like a tree planted by the streams of water. All of this is a gift from a gracious God.