The Chapter appears to be divided into three parts, all of which are related in one way or another to the human faculty of speech: Passuk 1, P'sukim 2-4, and P'sukim 5-7.
In the first part, David is expressing the awesome power of "Tefila," Prayer, and also the fact that it is possible for some human beings to assume the role of evil oppressors, and other human beings to assume the roles of innocent sheep. That HaShem is always on the side of the "underdog," as it says in Kohelet (3:15), "And the L-rd will intervene in behalf of the oppressed." And that HaShem has assigned tremendous power to the human being to invoke Divine aid, by means of prayer, as he says, (Tehilim 120:1), "When I was in distress, I cried out to G-d, and He answered me."
In the second part, David, who understands that ultimately, (Tehilim 116,11), "…all men are liars," nevertheless we should seek HaShem's aid in keeping us away from lies and falsehood. A particular scourge is "lashon hara," "evil speech" which is the utterance of words that will harm another, whether true or false. Tragically, it was this abuse of our power of speech that played a central causative role in the destruction of the Second Temple. These words are like "sharp arrows," inflicting injury at a distance, allowing the one who shot the arrows to pretend, and even to believe, that he was only practicing archery. We ask HaShem to protect us from falsehood and from hurtful speech.
Finally, in the third part, we ask, with David, for HaShem to protect us from our enemies, especially the ones we are dealing with now, late in the time of our Exile; namely, the descendants of Yishmael, "who dwell in the tents of Kedar," our present "peace partners." For them, falsehood is reality. They wish only to destroy us, and even when "I am for peace, and wish to negotiate, they are for war" (Tehilim 120:7).
period of the First "Beit HaMikdash," the Holy Temple