The end of chapter 16 told us that Alma traveled, but not the specific places he visited. This verse locates him as leaving Gideon and traveling south to Manti. In Gideon he had previously had a pleasant visit preaching to already-diligent believers (Alma 6–7). Presumably this subsequent visit was not generated by any concern about the Gideonites’ spiritual state, so it must have been on his way to Manti from wherever else he had been.
In any case, as Alma travels toward Manti, he meets his friends, the sons of King Mosiah, returning from their lengthy sojourn in Lamanite heartland. Manti is a logical meeting place, as the city apparently stands guard on the main route between the land of Nephi (the principal land of the Lamanites) and the land of Zarahemla.