Now therefore, O kings, be wise; be warned, O rulers of the earth.  Serve the LORD with fear, and rejoice with trembling.  Kiss the Son, lest he be angry, and you perish in the way, for his wrath is quickly kindled. Blessed are all who take refuge in him. (ESV)
This Psalm began with a question: Why do the nations rage and the peoples plot against God? Why do the kings and rulers take counsel against the Lord and his anointed one? This Psalm ends with a warning to these rulers of the earth. They are to submit to the Lord's authority and serve him. As a symbolic act of submission they are to kiss the Son.
The Son would be the representative king of God's kingdom. In the time of this Psalm's writing it would have been King David, the righteous king of Israel. Ultimately, we can view this Son as the "greater David," Jesus, God's eternal Son. And as such, it would be fair to recognize that all should humbly bow before Jesus, as the King of all Kings.
 Therefore God has highly exalted him and bestowed on him the name that is above every name,  so that at the name of Jesus every knee should bow, in heaven and on earth and under the earth,  and every tongue confess that Jesus Christ is Lord, to the glory of God the Father. (ESV)
Many people seem to want to believe in a God who is always loving, and never judging. The problem is that we do not get to pick and choose what characteristics we think God should have. His character is both loving and just. Those who rebel against him and reject his son will be judged eternally. Those who submit to his Lordship and "kiss the Son," will have eternal life.
What is your posture towards the God of all Creation? Are you raging against his supreme authority? Or have you kissed the Son? Only in submission to him is there life. And that life is wonderful.