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 126:1To praise for God's favor to His people is added a prayer for its continued manifestation. (Psa 126:1-6)
When the Lord, &c.--The joy of those returned from Babylon was ecstatic, and elicited the admiration even of the heathen, as illustrating God's great power and goodness.
turned again the captivity--that is, restored from it (Job 39:12; Psa 14:7; Pro 12:14). HENGSTENBERG translates: "When the Lord turned Himself to the turning of Zion" (see Margin), God returns to His people when they return to Him (Deu 30:2-3).
psa 126:4All did not return at once; hence the prayer for repeated favors.
as the streams in the south--or, the torrents in the desert south of Judea, dependent on rain (Jos 15:9), reappearing after dry seasons (compare Job 6:15; Psa 68:9). The point of comparison is joy at the reappearing of what has been so painfully missed.
psa 126:5As in husbandry the sower may cast his seed in a dry and parched soil with desponding fears, so those shall reap abundant fruit who toil in tears with the prayer of faith. (Compare the history, Ezr 6:16, Ezr 6:22).
psa 126:6He that goeth forth--literally, better, "He goes--he comes, he comes," &c. The repetition implies there is no end of weeping here, as there shall be no end of joy hereafter (Isa 35:10).
precious seed--rather, seed to be drawn from the seed box for sowing; literally, "seed-draught." Compare on this Psalm, Jer 31:9, &c.