With a Geodude (or another Pokémon with 'Crush') in your party, approach the large boulder blocking the bridge. Tap the Geodude's icon on the bottom screen, then tap the boulder. The Geodude will use its 'Crush' Field Move to smash the boulder, clearing the path. Once the boulder is gone, the construction worker will be able to proceed with repairs.
After the bridge is fixed, you can cross to the next area. Report back to the Ranger Union in Ringtown to complete Mission 2. This mission teaches you the importance of using the right Pokémon for the right obstacle, a core mechanic you'll use throughout the game.