The boulder is an access point to the God Wars Dungeon, found north of the Trollheim mountain. A Strength level of 60 is required to move the boulder, allowing you to pass - you will receive 1 damage in the progress. The boulder allows access to God Wars Dungeon for people without the necessary Agility level to use the Little crack shortcut next to it. The fastest way to get to the boulder is to use the Trollheim Teleport (Trollheim teleport (tablet)), descending the mountain either by using the various rock shortcuts or following the trail down, then going north-northeast until you reach the boulder.
BEWARE that the north part of the mountain is covered with Thrower trolls throwing rocks at you as you pass, it's advised to use Protect from Missiles as you pass.