Albert Barnes Commentary Daniel 4:26

"And whereas they commanded to leave the stump of the roots of the tree; thy kingdom shall be sure unto thee, after that thou shalt have known that the heavens do rule." — Daniel 4:26 (ASV)

And whereas they commanded—the watchers, (Daniel 4:15). Compare Daniel 4:17.

To leave the stump of the tree roots—or, to leave roots to the stump of the tree; that is, it was not to be dug up or wholly destroyed, but vitality was to be left in the ground. The Chaldee here is the same as in Daniel 4:15, leave the stump of his roots.

Thy kingdom shall be sure unto thee—that is, you shall not die under this calamity, but after it has passed away you shall be restored to authority. It might have been supposed that this meant that the authority would survive in his family, and that those who were to succeed him would reign—as shoots spring up after the parent tree has fallen—but Daniel was directed to an interpretation that is not less in accordance with the fair meaning of the dream than this would have been.

After that thou shalt have known that the heavens do rule—that is, that God rules. This was the great lesson which the event was designed to teach; and when that lesson had been learned, it would be proper for him to be restored to his throne and to proclaim this to the world.