A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 120:2This is the first of fifteen Psalms (Psalms 120-134) entitled "A Song of Degrees" (Psa 121:1 --literally, "A song for the degrees"), or ascents. It seems most probable they were designed for the use of the people when going up (compare Kg1 12:27-28) to Jerusalem on the festival occasions (Deu 16:16), three times a year. David appears as the author of four, Solomon of one (Psa 127:1), and the other ten are anonymous, probably composed after the captivity. In this Psalm the writer acknowledges God's mercy, prays for relief from a malicious foe, whose punishment he anticipates, and then repeats his complaint. (Psa 120:1-7)
Slander and deceit charged on his foes implies his innocence.
tongue--as in Psa 52:2, Psa 52:4.
psa 120:4Sharp arrows of the mighty--destructive inflictions.
coals of juniper--which retain heat long. This verse may be read as a description of the wicked, but better as their punishment, in reply to the question of Psa 120:3.
psa 120:5A residence in these remote lands pictures his miserable condition.
psa 120:6While those who surrounded him were maliciously hostile, he was disposed to peace. This Psalm may well begin such a series as this, as a contrast to the promised joys of God's worship.