This Psalm is the first of the fifteen Songs of Ascents. They were sung as the Jews went up to Jerusalem to worship, hence the references to the city, Zion, the house of the Lord, and the hills surrounding the city. They are songs of fellowship, as the people shared both the journey and the worship. The Psalms fall into five groups of three, with each group sharing common themes.
'In the capital of each district (of which there were 24) assembled those who were to go up with the first-fruits to the Temple. Though all Israel were brethren, and especially at such times would have been welcomed with the warmest hospitality each home could offer, yet none might at that season avail himself of it. For they must camp at night in the open air, and not spend it in any house, lest some accidental defilement from the dead, or otherwise, might render them unfit for service, or their oblation unclean. The journey was always to be made slowly, for the pilgrimage was to be a joy and a privilege, not a toil or weariness.' (Alfred Edersheim, The Temple, its Minstry and Service)
The problem of being surrounded by the ungodly. This Psalm begins the journey to Jerusalem. Some Jews, beginning that journey, were conscious of the ungodly around them. God had called the Jews to be a distinctive people, and this Psalm reflects that desire, but also the trial that such a desire would bring.
v1: The distress of being in the presence of the ungodly; in this situation the psalmist cries out to God.
v2: His prayer for deliverance. The particular concern was their words of lies and deceit; such things can damage, and bring accusation. We do not underestimate the damage that words can do.
v3-4: Judgment upon the ungodly. Ungodliness is evidenced by deceitful words; Matt 12.37.
v5: Meshech and Kedar, see Gen 10.2; 25.13; Is 60.7; Jer 49.28; Ezek 27.13; dwelling with the ungodly, who hate peace. Kedar was Arabia, where Ishmael's family lived.
v6-7: The problem of conflict, the man of peace living among those who hate peace. Indeed, the people of Ishmael were warlike, Gen 16.12.