As the name of the season suggest, it will adapt the manga's Hashira Training Arc, which covers chapters 128-136 of the manga, and is the arc before the manga's Final Battle. In the arc, Tanjiro goes to see the Stone Hashira, Himejima who intends to prepare him for the battles to come. The training to become a Hashira is intense and demanding, but Tanjiro won't give up until he earns Himejima's approval! Meanwhile, the demon lord Muzan continues to search for Nezuko and Ubuyashiki's location.
[Arc summary from Demon Slayer Wiki]