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.
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.
Use the HM move Strength.
you mest get the hm strength.
With the HM move strength.
use strength
move the boulders to where the lake Pokemon are
You get the hm strength and give it to one of your pokemon
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.