In the Gettysburg Address, Lincoln reflects on the soldiers that lost their lives in that field. He states that the soldiers sacrificed their lives in order to give us the country we now enjoy, and that in doing so, they have made the ground sacred.
He emphasizes the importance of this in order to urge those who are still living to protect their country. Lincoln tells us that it is up to us to continue the work that they began, so that their sacrifice is not in vain.