Why was Martin Luther King’s “I Have A Dream” speech so pivotal in American history? Did it redefine the nature of America and of the American Dream?

Martin Luther King, Jr. delivered his famous "I Have a Dream" speech on August 28, 1963, during the March on Washington for Jobs and Freedom. His speech was pivotal because it brought civil rights and the call for African-American rights and freedom to the forefront of Americans' consciousness.
It is estimated that over 250,000 people attended the march, which also received a great deal of national and international media attention. The speech helped make Civil rights an issue that attracted by African American and white people in a national coalition. In the speech, Dr. King promoted his idea of non-violent resistance as a means of achieving equality and stated, "We must not allow our creative protest to degenerate into physical violence." His words helped gain passage of the Civil Rights of 1964, which provided for equal rights for African-American people. In addition, the speech was one of the most eloquently written and most movingly delivered speeches in American history. It is still studied today as an example of fine rhetoric.
Dr. King gave his speech in front of the Lincoln Memorial, which was symbolic because Lincoln had signed the Emancipation Proclamation in 1863, exactly one hundred years before, freeing the slaves in the Confederacy. Dr. King mentioned this symbolism in his speech, stating, "Five score years ago, a great American, in whose symbolic shadow we stand today, signed the Emancipation Proclamation." He went on to say, however, that "the Negro is still languished in the corners of American society and finds himself in exile in his own land." By speaking in front of the Lincoln Memorial, Dr. King contrasted the majesty of the memorial with the reality of the condition of African Americans in the country, who were still disenfranchised and treated like second-class citizens at best.
In his speech, Dr. King called for equality and redefined the American Dream not as one of prosperity alone but as one of freedom and equality. In a series of passages that began with the words "I have a dream," King stated his vision of equality and brotherhood in all parts of the United States. For example, he stated:

"I have a dream that one day on the red hills of Georgia, the sons of former slaves and the sons of former slave owners will be able to sit down together at the table of brotherhood."

Dr. King redefined America not as a place in which only white people were entitled to the promises of liberty and freedom but as a place in which everyone had the right to these promises and to the potential for acceptance and equality. He redefined the American Dream not merely as the right of whites to access life, liberty, and the pursuit of happiness but as the right of all people to do so and to live with the full acceptance of others. He believed that Americans could not be free until everyone was free, and so he redefined the American Dream as not only an individual promise but as a communal promise as well. 
https://kinginstitute.stanford.edu/king-papers/documents/i-have-dream-address-delivered-march-washington-jobs-and-freedom