In my Bible, Psalm 19 has a subtitle, "God's Glory in Creation and the Law," which about sums it up!
A parallel is drawn by the Psalmist (surprise! David again!) between the revelation of God through nature and the beauty of His law. These two emphases neatly divide the psalm. God's message as it can be discerned through Creation is to the world. His unique word to Israel comes through their special possession of the Law. Isn't it great to have both testimonies, and not be forced to choose between having one or the other?
Side note: as a child, every Sunday morning in church I would hear our preacher pray, "May the words of my mouth and the meditation of our hearts be acceptable in your sight, you who are our rock and our redeemer," right before the sermon. Left an impression!