What does Psalm 6:4 mean?

David assumes the Lord has turned away from him. We're not sure if this is because of some sin he committed, or because of some desperate condition he is facing. It might be both, such as David experiencing an illness which he knew was connected to his sin. In response, he asks God to turn back and rescue him.

This plea is not based on any merit which David claims for himself. Instead, it's entirely rooted in the Lord's steadfast love. He realizes that the covenant-keeping Lord God loves His people regardless of their sins or adverse circumstances.

After making a second set of tablets and being commanded by the Lord, Moses arose early and ascended Mount Sinai with the tablets in hand. There the Lord met him and proclaimed, "The LORD, the LORD, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness, keeping steadfast love for thousands, forgiving iniquity and transgression and sin" (Exodus 34:6–7). Perhaps David recalls these words from Exodus as he appeals for deliverance.