The importance of Martin Luther King, Jr.'s “I Have a Dream” speech is...

Martin Luther King's "I Have a Dream" speech, delivered on the mall in Washington, DC, in front of an enormous audience, was one of the defining events of the civil rights movement.
Its importance is manifold. King begins with a reference to the Emancipation Proclamation, the executive order issued by Abraham Lincoln in 1863 that freed the slaves. He remarks, sadly, that "one hundred years later, the Negro is still not free." The speech was the crowning event in the march on Washington, a major protest organized by civil rights leaders in the battle to improve the economic and social conditions of African-Americans.
King was a preacher and master orator, but the "I Have a Dream" speech stands head and shoulders above much of what he said before. Apparently the final passages of the speech, where King compares his dreams about the success of the struggle for civil rights with the larger idea of the "American dream," was not prepared beforehand and is an ad-lib.
Whether prepared or not, it is clear that King's words were a powerful rallying point and may have tipped the balance toward the final signing of the Civil Rights Act in 1964. It is, without doubt, one of the great speeches in American history, comparable even to Lincoln's Gettysburg Address.