Lincoln delivered this speech during the American Civil War, on Thursday, November 19, 1863, during the afternoon, at the dedication of the Soldier's National Cemetery in Gettysburg, Pennsylvania. The address was made four and a half months after the defeat of the Confederate armies by those of the Union at the Battle of Gettysburg.
To consecrate means to declare something holy and hallow is its synonym. Lincoln is saying that the ground cannot be declared holy, because:
The brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract.
Lincoln means that the extreme sacrifice the men who had fought and died on that battlefield have made was in itself a greater act than any other could now, or ever, perform in ordaining the soil on which they died. Their deed was more than enough for the ground to be consecrated. The blood that was spilt there blessed the soil. It would, he suggests, be presumptuous of him or any other to believe that they could do those who gave their lives greater honor by declaring the ground hallowed.
These words indicate the great respect Lincoln had for those who gave up their lives to fight for a noble cause. He wanted them to be honored through more than symbolic gestures such as this one—he felt they should be held in esteem in the hearts and minds of all Americans. That should be how a nation conveys its greatest gratitude.