English Basic Explanation - Chapter 120
Psalm 120 begins a series of fifteen chapters which open with the words "Shir Ha'ma'alot" (or, in chapter 121, "Shir La'ma'alot"), which is generally translated as "A song of ascents." A number of commentators cite from the Talmudic sages that these fifteen songs were sung by the Levi'im in the Bet Ha'mikdash as they ascended the fifteen steps that separated the two sections of the courtyard outside the Temple. Each time they ascended one level, they would sing one of these Psalms, until at the final step they sung the fifteenth chapter of this series.
This chapter is a prayer for protection from dishonesty and falsehood. Am Yisrael has always had to confront not only powerful armies, but also false charges and accusations. Their enemies would often fabricate information to discredit them and win support in their campaign to destroy the Jewish people. In this chapter, Am Yisrael laments the hardships it has suffered at the hands of such nations, who respond to their kind, peaceful gestures with hatred and hostility.
In verse 4, the author likens falsehood to "sharp arrows of warriors." Arrows, as Rashi explains, are used to kill enemies at a distance. Dishonesty works the same way, killing reputations and even people from a distance. False rumors and accusations can destroy a person even if the speaker is nowhere near the victim, and allows one to "shoot down" his adversary without having to confront him directly. These tactics are thus especially dangerous, and, as this Psalm expresses, it is only with God's help that Am Yisrael can protect itself from these destructive "arrows."