Tell me not, in mournful numbers,
Life is but an empty dream!
For the soul is dead that slumbers,
And things are not what they seem.
Life is real! Life is earnest!
And the grave is not its goal;
Dust thou art, to dust returnest,
I.e., don’t except things to be the way you expect them to be. The “dream / seem” rhyme is classic Longfellow; didactisicm by poetic flourish. 

A Psalm of Life