On the preceding verses, Malbim has explained that this chapter of Tehillim is in the voice of a Levi who is in exile in Bavel, and who longs desperately to once again return to the Beis Hamikdash and resume singing G-d's praises there. Meanwhile (as in Tehillim 137), his harp is hanging mutely on the willows in Bavel.
On verse 6 (מסילות בלבבם), then, he explains that those who fervently look forward to Hashem's salvation have "paved roads in their heart" as the route by which they will return to Tziyon. And so in v. 7 he continues:
Those who pass: These paved roads in their hearts "pass through the Depths of Bacha," as Bavel was a lowland full of becha'im [often identified as mulberry] and [other] trees; indeed, it is called "the valley of the willows" [Yeshayah 15:7; see Malbim's commentary ad loc.]. Instead of willows, the poet chose to use the term "becha'im," which also suggests bechi (crying), since "by the willow trees in it" they sat and wept [Tehillim 137:1-2]. These paved roads pass through the becha'im, and with weeping "they will go to G-d in Tziyon," for their heart pictures them traveling on the roads through the becha'im to Tziyon.
They make it a fountain: These paved roads in their hearts make their hearts into a fountain filled with a bounty of water and blessings, to the point where their hearts' thoughts and visions are like an ever-increasing fountain.
Also the late rain enfolds with blessings: The blessings in which the heart is enfolded by his visions, and his great trust [in G-d], are the means by which he provides rain and showers to others, for from his fountain of blessings arise clouds that dispense bountiful showers, to provide rainfall and blessing to all of Israel, filling their hearts with hope and expectation that they will return to their stronghold.
