refers to Reconstruction itself. Unlike previous (white) historians, DuBois characterized Reconstruction as an important, albeit brief, moment in which African Americans had an opportunity to achieve full equality. All men were given the right to vote by the Fifteenth Amendment, and the opportunities created by organizations such as the Freedmen's Bureau and others gave them what seemed at the time a chance at social equality. These gains were always tenuous, however, always threatened by the economic realities of the South, as well as the actions of politicians and terrorist groups, such as the Ku Klux Klan. So the "moment in the sun" depended on federal support for Reconstruction.

After the end of Reconstruction, southern states established a new, harsh system of racial stratification known as "Jim Crow." Black men were stripped of the right to vote through a number of methods, rigid laws were set up to maintain separation between the races, and society, in short, was deliberately structured in such a way as to establish and maintain white supremacy. From the post-Reconstruction era to the 1930s—when DuBois wrote—thousands of black men were lynched for allegedly violating race laws and other rules. So from the standpoint of the 1930s, it looked indeed as if African Americans had "moved back again toward slavery."

In this quote, Du Bois refers to the Emancipation Proclamation, passed in 1863, which freed slaves in the South. The Emancipation Proclamation later became the 13th Amendment in 1865, which freed the slaves nationwide. The freed slaves "stood for a brief moment in the sun" when they enjoyed the early gains of the Reconstruction Era, including the passage of the 14th Amendment, which granted them citizenship, and the 15th Amendment, which granted African-American men the right to vote. During the early years of Reconstruction, freed slaves enjoyed some voting rights and representation in local and federal government and some degree of land ownership. However, freed slaves "moved back again toward slavery" when Southern states began passing voting restrictions for African-Americans through means such as the poll tax and literacy tests. In addition, economic freedom for African-Americans became limited, and many were forced back onto plantations as sharecroppers who worked the land for white farmers but didn't own the land. 

In tracing the development of Black consciousness in America, DuBois focuses on the period of slavery, which helped to create the stigma or problem of "color," the Civil War with Abolitionists and Lincoln, and the challenges of Reconstruction.  With the ending of the Civil War, the North's victory, and the official end to the institution of slavery in the South, the "slave went free."  Slavery was over and there was a feeling of freedom being present.  This represented "a brief moment in the sun."  The establishment of the Freedman's Bureau helped to "make the Negro a ward of the nation."  Noting its successes in the South, in particular educational opportunities and the expansion of health care and other government services, this would be the moment where the sun' moment was felt.  The brevity or shortness of this moment was valid and real as there was no political power for people of color (14th Amendment not yet passed), social inequality still continued in that many Southern Whites were not willing nor comfortable with full social recognition of African- Americans, and this translated to a lack of economic empowerment opportunities.  Both of these components- the lack of a political and economic voice- represented the "move back again towards slavery."  The quotation reflects the complexity in achieving a valid conception of freedom, a complexity that was manipulated to ensnare African- Americans in the wake of the Civil War.