In this speech, Lincoln discusses how the Southern states (aka the Confederacy) will be received back into the Union.
This era of the Confederate States rejoining the Union is known as the era of Reconstruction. Lincoln gives this speech after the surrender of the Confederate forces to discuss his initial reactions to the war being over and the challenges that lie ahead. However, he will not live to see this process as he is assassinated three days later by John Wilkes Booth.