Explain "and things are not what they seem"

This line comes from the poem "A Psalm of Life" by Henry Wadsworth Longfellow. Longfellow wrote the poem as a sort of response to the belief, often expressed by the Psalms in the Bible, that this life does not matter and that men should focus only on the afterlife. He starts the poem with the speaker directly addressing the writer of the Psalms, saying, “Tell me not, in mournful numbers/Life is but an empty dream!” (1-2). The speaker does not want to accept the message that life is meaningless, therefore, when he says “And things are not what they seem” (4), he sends a hopeful message that “life is real” (5). To say “things are not what they seem” is essentially to question the message in the Psalms and to say that there is hope for life. For the rest of the poem, he supports this by giving people advice about how to live life in the present rather than to sit by and watch life pass them by.