"The Man with the Hoe" is a poem that questions the power differential in society and how those with power treat those without.

To understand the second stanza, one must start with the epigraph: 

God made man in His own image, 
in the image of God made He him. (Genesis)

Following this biblical reference, the first stanza paints the image of a downtrodden farmworker. There is a striking contrast presented by these two ideas. The epigraph presents the idea that humans are made in the image of god, or that we are like god (or divine) ourselves. The second presents a very un-deified image of humanity: an overworked farmworker, dead in his tracks.

The second stanza confronts the reader with the discrepancy between the two. Consider the exasperated first line: "What gulfs between him and the seraphim!" Here, the author is pointing our attention to the distance between the dead farmer and the seraphim, or angels. The list of rhetorical questions that follows underscores the joyful aspects of life that the farmer is deprived of due to his socioeconomic status: music, the loveliness of the sunrise, the blossoming of flowers.

From there, the narrator steps back to compare the dead farmer to all of us: 

Through this dread shape the suffering ages look;

Time’s tragedy is in that aching stoop....   

The meaning of the second stanza of the poem is that the working person is being treated in an ungodly way--in a way that God never intended. While God created people to rule over the land and seas, to search the heavens, and to contemplate eternity, the working person lives in a way that is degraded and different from God's design for him or her. While God's dream was to create people as exalted beings, people live in a way that is worse than how creatures in hell live. The state of the working person is a living critique of the world's greed and of omens of bad things to come. In essence, the way working people, such as farmers, are treated in is direct contradiction to God's design for them. 

What is the meaning of the third stanza of "The Man with the Hoe"?

Markham's "Man with a Hoe" was inspired by the the Millet painting of the same name. Markham's poem is, therefore ekphrastic, or a piece of literature about a work of art.

Millet's "Man with a Hoe" depicts an exhausted, almost animalistic looking man bent over his hoe. In the poem, written at at time of great wealth inequality in 1898, the poem's speaker wonders why the very wealthy so degrade and exploit poor people, such as this man. Surely they have so much wealth that they could treat the workers better than this.

In the third stanza, the speaker exclaims over the great distance that separates this man from heavenly angels (seraphim). He asks how a person so enslaved to labor can possibly enjoy the highest achievements of humankind, such as the works of Plato? He wonders when the man can have the time to look at a constellation such as the Pleiades.

The speaker in this stanza calls it a "tragedy" that this man and others like him have been "betrayed" by exploitation and deprived of their humanity—the time and energy to devote to taking pleasure in natural and artistic beauty. He says this man has been robbed of his rightful human legacy. Finally, the speaker ends by stating that this exhausted figure is a cry of protest against the powerful people in the world who have allowed this to happen. The man is also a prophecy or foretelling of the inevitable rise of such exploited souls against those who held them down.