1 Understand, beloved, this Psalm, which I am about to explain, by the grace of God, of our hope in the Lord Jesus Christ, and be of good cheer, because He who promised, will fulfil all, as He has fulfilled much: for it is not our own merit, but His mercy, that gives us confidence in Him. He Himself is meant, in my belief, by “the understanding of Æthan the Israelite:” which has given this Psalm its title. You see then, who is meant by Æthan: but the meaning of the word is “strong.” No man in this world is strong, except in the hope of God's promises: for as to our own deservings, we are weak, in His mercy we are strong. Weak then in himself, strong in God's mercy, the Psalmist thus begins: “I will sing of Your mercies, O Lord, for ever: with my mouth will I make known Your truth unto all generations”.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)