So, what does it mean?

Thus, “righteousness” does not mean moral perfection. It is not a distant, forbidding characteristic of God that humans are supposed to try to emulate or imitate; there is no good news in that. Instead, the righteousness of God is God’s powerful activity of making right what is wrong in the world. When we read, in both Old and New Testaments, that God is righteous, we are to understand that God is at work in his creation doing right. He is overcoming evil, delivering the oppressed, raising the poor from the dust, vindicating the voiceless victims who have had no one to defend them.—Fleming Rutledge, The Crucifixion, 328 (all emphasis original)