Lincoln is alluding to the Declaration of Independence in the opening of his Gettysburg Address:
Four score and seven years ago our fathers brought forth on this continent, a new nation, conceived in Liberty, and dedicated to the proposition that all men are created equal.
He doesn't have to tell his audience this is the document he means because of all the clues in the above statement. A score is twenty years, so four score years plus seven would have been eighty-seven years. In 1863, that would have been 1776, the year the Declaration was penned.
Further, Thomas Jefferson's document declared the United States a new nation, separate from England. Finally, the Declaration is dedicated to the idea that all men, at least all white men, are created equal.
Lincoln's referencing of the Declaration of Independence, a foundational text in US, history, to commemorate the decisive Union victory at the Battle of Gettysburg would have created a warm emotional feeling in his listeners. Lincoln is reminding his audience of what they are fighting for and the sacrifices their forebears made to create a new nation. He goes on to state that the North is waging this war so that the nation the founders struggled to establish can "live." He urges the troops and other listeners not to lose devotion to the cause of preserving the nation, so that they will make sure to keep on struggling to the end, ensuring that those who died in Gettysburg did not die in vain.