This is a song about contentment. In verse two David says he is content in God's presence like a small child is content in the presence of his mother (Ps 131:2). He has weaned himself away from incessantly demanding answers and things from God and instead just rests in the presence of God.
This all began with an attitude of humility willing to let God handle the unknown. David did not demand from God what he did not know. He chose to let those deeper mysteries remain with God and in God's timing. David realized his limitations and leaned on God.
This contentment was David's hope. From it he calls all of Israel to always hope in God (Psalm 131:3). David had learned humility, simple joy in trusting God rather than demanding from God, and he called others to know the confident hope of a contented son.