What does Psalm 103:8 mean?

The thought of what God had done for Israel led David to celebrate His grace, patience, mercy, and consistent love. But David also knew these facts about the Lord through personal experience. God had shown David mercy and unmerited favor by withholding death from him after David's affair with Bathsheba (2 Samuel 12:7–13). David had confessed his sin, and the Lord forgave him and demonstrated His love toward him (Psalm 51:1–3).

The nation of Israel, as well, had experienced the Lord's mercy and grace. Despite frequent rebellion against the Lord, the people of Israel had found forgiveness and assurance that the Lord had set His love upon them. They had learned the truth God proclaimed to Moses on Mount Sinai: "The LORD, the LORD, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness" (Exodus 34:6). Time after time the nation's people provoked the Lord, but He did not destroy them. The fact that Israel exists today bears testimony to the Lord's mercy, grace, and love.