The first section of the manual gives a nice introduction to the lesson and its subject. I would start by reading it verbatim.
The Atonement Is Necessary for Our Salvation
Jesus Christ “came into the world … to be crucified for the world, and to bear the sins of the world, and to sanctify the world, and to cleanse it from all unrighteousness; that through him all might be saved” (D&C 76:41–42). The great sacrifice He made to pay for our sins and overcome death is called the Atonement. It is the most important event that has ever occurred in the history of mankind: “For it is expedient that an atonement should be made; for according to the great plan of the Eternal God there must be an atonement made, or else all mankind must unavoidably perish; … yea, all are fallen and are lost, and must perish except it be through the atonement” (Alma 34:9).