As clarified in verse 2, Mormon is using Alma’s personal record for this chapter. This personal record will be the source of all but one chapter, from here to our Chapter 43, which returns to the large plate record.
Mormon will typically quote heavily from Alma, where he typically summarized material from the large plates. To set up the quoted material, he gives this short introduction. Alma intends to heal his people through repentance and a refreshed commitment to following Jehovah’s gospel. He will do this by preaching, and he begins in the main city, Zarahemla.