The move you are thinking of is Strength, and you get it from the Safari Zone Warden in Fuchsia City. To receive it, you need to return to him his Gold Teeth, which you find deep within the Safari Zone.
No Pokemon can move boulders by themselves, they have to learn a Hidden Move (HM) called Strength. You get this HM by giving the Warden in Fushia City his Gold Teeth back. You can find his teeth in the Safari Zone, close to the "Secret House". The Warden lives at the east-side of Fushia in a house with a pond behind it.
The HM04 Strength. This is given to you after you retrieve the gold teeth from the safari zone and give them to the warden.
First, you have to get the HM strength, then teach it to and a Pokemon and you can move boulders.
Strength
How do you move boulders in Pokemon:LEAF GREEN - FIRE RED - YELLOWTo move boulders you have to learn a HIDDEN MOVE (HM)called STRENGTH.You get this HIDDEN MOVE (HM) by giving the Warden in Fushia City hisGold Teeth back.You can find his teeth in the Safari Zone, close to the Secret House.The Warden lives at the east-side of Fushia in a house with a pond behind it.
To move boulders in Pokemon Gold you need a Pokemon with the HM move Strength. Selecting the Pokemon in the menu outside of battle should let you use it if you have beaten the Olivine City Gym.
you mest get the hm strength.
Use the HM move Strength.
With the HM move strength.
use strength
You get the hm strength and give it to one of your pokemon
move the boulders to where the lake Pokemon are
Moltres makes its roost on the peak of Mt Ember. HM04-Strength is needed to move boulders to reach the summit.
Move the boulders around the place where the lake Pokemon are.