I assume you are referring to the visit by the "Confederate" soldier, who is really a spy for the Yankee army. This part is told in flashback, going back in time so the reader knows how Farquhar came to be standing on the bridge with a noose around his neck. Farquhar, a plantation owner, is unable to fight in the war, but he still wants to prove himself worthy as a Southerner. When he asks about the war (Civil War), the soldier tells him that the Yankees had built a fort near the bridge and were repairing the railroads. The soldier suggests to Farquhar that the burning of Owl Creek Bridge would set back the Yankees' plans to move forward. Farquhar, wanting to show how brave he is, decides he will burn the bridge to stop the Yankees.