When Lincoln says that the soldiers who fought or died on the battlefield "have consecrated it far above our poor power to add or detract," he means that their actions and sacrifices have given a special, sacred meaning to that place that surpasses anything the living can do or say. The soldiers' devotion, bravery, and willingness to lay down their lives in defense of their cause have elevated the battlefield to a higher level of significance.
In essence, Lincoln is emphasizing the profound and enduring impact of the soldiers' actions and the lasting significance of their sacrifice. He seeks to honor their memory and acknowledge that the living must strive to live up to the ideals for which they fought and died.