Man is like to vanity - See the notes at Psalms 39:5-6; Psalms 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 Psalms 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.