"Five score years ago, a great American, in whose symbolic shadow we stand today, signed the Emancipation Proclamation. "
In the second paragraph he is referring to Lincoln's Emancipation Proclamation which was made a hundred years before King's speech, "I Have a Dream." Lincoln was a huge contributor to the fight against discrimination.
An allusion is a literary device where the speaker or the author refers directly or indirectly to a well-known person, place, event or literary work. In this case it was used to contribute to the message of what he is conveying.
