There are two - and only two - kinds of people in the world. In the words of this psalm, they are "the wicked" and "those who know You." David wants to remind the people, those who come early in the morning to the tabernacle, of these two kinds of people. He wants them to be in the second category. He wants them to pray for the kind of light for daily living that can come only from God and from knowing God.
He begins by singing a summary description of "the wicked." He says that "there is no fear of God before his eyes." It's the reason they are the wicked, the reason, in fact, for everything they are. It answers the question, "Why are they like that?" They are the way they are because God means nothing to them.
The way the wicked see it, there isn't anyone watching them - at least anyone concerned about their behavior. Simply put, he doesn't think there's a God Who cares. David says that the wicked "flatters himself in his own eyes concerning the discovery of his iniquity." For that reason, "He has ceased to be wise and to do good. He devises wickedness on his bed; he sets himself in a way that is not good; he does not abhor evil."
That's the way it is with the relativism of our own day that says there's no God, so every man is therefore his own standard - what's right for you is right for you, and what's right for me is right for me.
But there really IS a God Who cares! And those who know Him love His mercy and faithfulness, His righteousness and judgements, and His providence over all His creation. They have discovered His loving kindness and have put their trust in it. They're "abundantly satisfied with the fullness" of that loving kindness, and they drink of the "river of Your pleasures" - the joy of knowing and loving that true God. Not only so, but they've even found that they can actually understand life, because, they say, "with YOU is the fountain of life - in Your light we see light."
After the people have reaffirmed their faith by singing these words, then they're ready to pray. So David leads them in the prayer of those who truly know God. He says, "Oh continue Your loving kindness to those who know You, and Your righteousness to the upright in heart." He prays that they might actually experience their God in these ways this very day! Their prayer thus seeks continuing realization of the Lord's reality, blessing, and care in their lives.
They close by praying to be kept from the influence of unbelievers, lest they fall. It's like one of the things found in the prayer our Lord taught His disciples. In that prayer they are to say, Lord, "Lead me not into temptation, but deliver me from the evil one." This prayer does the same.
How do you want your day to go? Will you pray this prayer? Will you seek every day to know the presence and blessing of a real and present God?