Celestic Town is an ancient town with a rich history. Heal your Pokémon. You'll find a Team Galactic Grunt guarding the town's ruins. Talk to the elder, Cynthia's grandmother, who will tell you about the town's history. After talking to her, challenge the Team Galactic Grunt. After defeating him, Cyrus will appear, give another cryptic speech, and then leave.
Cynthia's grandmother will then give you HM04 (Strength). You can now use Strength outside of battle, which will be useful for moving boulders. After exploring Celestic Town, Fly back to Jubilife City and head west onto Route 218.