Asaph reminds the the Old Testament Jewish believer that God is coming to judge the earth and that judgment beings with those who put their trust in God. Remember the theme of this Psalm is about those who go through the motions of worshiping God, but their hearts are not in it.
God is a righteous judge who will judge all people. Verse 6 reminds us that even the heavens proclaim His righteousness, for God Himself is judge. Selah. The word "Selah" simply means to stop and think about what was just said. Some may ask, 'who or what in heaven is proclaiming God's righteousness?' A possible answer is the angels. They are proclaiming that God is "right" to judge people as He is at this time. The word "heavens" also refers to what is above the sky. The verse could be saying that God's creation is proclaiming His righteousness by just being evidence of His existence and the fact He rules over the world.Practically, all should live with an awareness that one day they will stand before this righteous Judge.
The God of Israel is preparing to speak to His people and make the charge in the verses that follow.