What does Psalm 19:13 mean?

The prior verse referred to "hidden sins," meaning those a person may commit without realizing it until later. Here, however, David prays the Lord will restrain him from committing willful sins, called "presumptuous sins" in this case. Willful sins are committed with the eyes wide open.

When David committed adultery with Bathsheba he was fully aware of what he was doing. He saw her and sent for her. He also willfully arranged for Uriah, Bathsheba's husband, to be killed in battle. He was, therefore, guilty of adultery and murder. Both acts violated obvious moral law and the explicit commandments God had given to Israel (Exodus 20:13–14). No sacrifice could atone for willful sins, but when David confessed his sins, God forgave him (Psalm 32:5; 51:16–17; 2 Samuel 12:13).

Committed often enough, willful sins can become addictive and enslave their victim. David affirms here that if willful sins did not rule him, he would be beyond reproach and great transgression. The original Hebrew language does not include the definite article "the" with "great transgression." Most likely, David does not have one specific sin in mind. If he did, perhaps, by "great transgression" David meant the sin of idolatry.