This is a very long speech but the gist of the speech is that white men received liberty and freedom in 1776. Independence from Britain happened for those who were not slaves and so the 4th of July is a reminder of the inequality and cruelty of the American promise.
The following is the most famous lines of the speech. It is about half way through his speech which he delivered to the President and fellow citizens on July 5, 1852.
"What, to the American slave, is your 4th of July? I answer: a day that reveals to him, more than all other days in the year, the gross injustice and cruelty to which he is the constant victim."