To understand the first stanza of this poem, or any stanza, for that matter, it is necessary to appreciate American history, particularly as it applies to the author and his ancestry.
In the United States, we celebrate our history of freedom, which stems from our freedom from our British masters. However, throughout the entire history of the United States, up to the point at which Hughes was writing, freedom was not available to everyone, in Hughes' case, to African-Americans, but also not really available to many poor white people, too.
First, there was slavery and indentured servitude until the end of the Civil War, in 1865. Subsequently, there were Jim Crow laws, laws created mostly by the southern states, which restricted the freedom of African-Americans, even though they were free as a matter of federal law. Few of them were able to eat where they wished to, ride trains in the same cars as white people, or use "white" public restrooms or hospitals, much less own property or vote. Poor white people lacked many freedoms as well, mostly because they were poor and/or uneducated and lacked the wherewithal to take advantage of the great opportunities America supposedly provided to all.
The first stanza of this poem paints a pretty picture of America as we would like to believe it was settled, the "pioneer upon the plain" (line 3), dreaming the American Dream, free to pursue his opportunities. It is superficially nostalgic, a longing for the days when people were free. But the line that follows the first stanza tells us that this was not the narrator's America at all, and that there can be no nostalgia for something that did not exist.
How can America be free "again" if it was never free for so many people in the first place? It was not free for Hughes' ancestors or for many poor white people, and in fact, Hughes, who lived from 1902 to 1967, did not experience the freedoms America has supposedly promised, since throughout most of his lifetime, he was subject to a culture and legal system in which he could be and was systematically discriminated against. The Civil Rights Act did not get passed until 1964, three years before his death.
Thus, this first stanza, which sets the tone for the entire poem, is meant to be a bitter reflection upon an America that never was, for many people, including Hughes himself.