Thou, O God, didst send a plentiful rain - Margin, shake out. Prof. Alexander, "a rain of free gifts." The Septuagint and the Vulgate render it, "a voluntary or willing rain." The Syriac, "the rain of a vow." The Hebrew word translated "plentiful" means free, voluntary, of its own accord - נדבה nedâbâh - (See the notes at Psalm 51:12, where it is rendered free); then it means that which is given freely; and hence, abundantly. It means, therefore, in this place, plentiful, abundant. The reference, however, is to the manna, with which the people were supplied from day to day, and which seemed to be showered upon them in abundance. The word rendered "didst send" means properly to shake out, as if God shook the clouds or the heavens, and the abundant supplies for their needs were thus shaken out.

Whereby thou didst confirm thine inheritance, when it was weary - Thou didst strengthen thy people when they were exhausted, or were in danger of fainting. In other words, God sent a supply of food - manna, quails, etc. - when they were in the pathless wilderness, and when they were ready to perish.

Psalm 68:9 Parallel Commentaries

That it is Profitable to Communicate Often
The Voice of the Disciple Behold I come unto Thee, O Lord, that I may be blessed through Thy gift, and be made joyful in Thy holy feast which Thou, O God, of Thy goodness hast prepared for the poor.(1) Behold in Thee is all that I can and ought to desire, Thou art my salvation and redemption, my hope and strength, my honour and glory. Therefore rejoice the soul of Thy servant this day, for unto Thee, O Lord Jesus, do I lift up my soul.(2) I long now to receive Thee devoutly and reverently, I desire …
Thomas A Kempis—Imitation of Christ

The Exile --Continued.
There are many echoes of this period of Engedi in the Psalms. Perhaps the most distinctly audible of these are to be found in the seventh psalm, which is all but universally recognised as David's, even Ewald concurring in the general consent. It is an irregular ode--for such is the meaning of Shiggaion in the title, and by its broken rhythms and abrupt transitions testifies to the emotion of its author. The occasion of it is said to be "the words of Cush the Benjamite." As this is a peculiar name …
Alexander Maclaren—The Life of David

Book iii. The Ascent: from the River Jordan to the Mount of Transfiguration.
{hebrew} In every passage of Scripture where thou findest the Majesty of God, thou also findest close by His Condescension (Humility). So it is written down in the Law [Deut. x. 17, followed by verse 18], repeated in the Prophets [Is. lvii. 15], and reiterated in the Hagiographa [Ps. lxviii. 4, followed by verse 5].' - Megill 31 a. …
Alfred Edersheim—The Life and Times of Jesus the Messiah