Albert Barnes Commentary Psalms 68:9

"Thou, O God, didst send a plentiful rain, Thou didst confirm thine inheritance, when it was weary." — Psalms 68:9 (ASV)

You, O God, sent 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 n e dâbâh — (See the notes at Psalms 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.

By which you confirmed your inheritance, when it was weary — You strengthened your 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.