Answer: he wants to point out that many do not have freedom.
In this excerpt, Douglass wants to point out the hypocrisy of celebrating the independence of America while many people remained slaves. Independence, by definition, means freedom, and when Americans celebrate their independence, they celebrate the freedom of not being dominated by another country. He argues that a large percentage of the population cannot understand this feeling, as they are dominated by other people. Therefore, the independence of the country is of little importance to those that still cannot enjoy freedom.
