You have to use a HM called strengh.
Chat with our AI personalities
You get the hm strength and give it to one of your pokemon
you first you beat a gym leader then you get the hm strength and teach it to a Pokemon then you push some boulders
Use Strength. It can be obtained at Olivine City in a building next to the Pokemon Center where there is a sailor inside. Teach your Pokemon Strength so that you can push aside big boulders.
use strength
HM04-Strength