Psalms 120 - Wesley's Explanatory Notes

Arrows — The wrath and vengeance of the mighty God, which in scripture is often compared to arrows, and here to coals of juniper, which burn very fiercely and retain their heat for a long time.

Mesech — Mesech and Kedar are two sorts of people often mentioned in scripture, and reckoned amongst the barbarous nations. But their names are here to be understood metaphorically. And so he explains himself in the next verse.