Man is like to vanity - See the notes at Psalm 39:5-6; Psalm 62:9. The idea here is, that man can be compared only with that which is utterly vain - which is emptiness - which is nothing.
His days are as a shadow that passeth away - See the notes at Psalm 102:11 : "My days are like a shadow that declineth." The idea is essentially the same. It is, that as a shadow has no substance, and that as it moves along constantly as the sun declines, until it vanishes altogether, so man has nothing substantial or permanent, and so he is constantly moving off and will soon wholly disappear.

Man is like to vanity - אדם להבל דמה Adam lahebel damah, literally, Adam is like to Abel, exposed to the same miseries, accidents, and murderers; for in millions of cases the hands of brothers are lifted up to shed the blood of brothers. What are wars but fratricide in the great human family?
His days are as a shadow - The life of Abel was promissory of much blessedness; but it afforded merely the shadow of happiness. He was pure and holy, beloved of his parents, and beloved of God; but, becoming the object of his brother's envy, his life became a sacrifice to his piety.

Man is like to vanity,.... Is vanity itself, in every age, state, and condition; yea, in his best estate, Psalm 39:5; or, "to the breath" (h) of the mouth, as Kimchi; which is gone as soon as seen almost: or, to a vapour (i); to which the life of man is compared, James 4:14;
his days are as a shadow that passeth away; as the former denotes the frailty and mortality of man, this the shortness of his duration; his days fleeing away, and of no more continuance than the shadow cast by the sun, which presently declines and is gone.
(h) "halitui", Muis; so Kimchi. (i) "Vapori", Cocceius; so the Syriac and Arabic versions.