"I have gained more understanding than my elders, for I observe Your precepts." This helps explain how the Jewish custom of visiting the tombs of tzaddikim and praying there, that their merit protect them and guard them from all trouble, distress, illness and the like.
The reason behind this custom is that deceased tsaddikim are said to have remained alive through their praying for life and peace for us, and that their prayer is more effective than ours…
Those whose prayers were answered because of a tzaddik's merit came pure of heart, strong in their repentance and broken-hearted, which is why their prayers were answered. This is what our lord King David meant, in saying "my elders" – referring to the tsaddikim, who are considered alive after their deaths; "I have gained more understanding" why the prayers of some are answered - because of their merit; and to defend protect and guard them from all sorts of disaster, because "I observe Your precepts" and am free of transgression.