In the simplest terms, to be wild. To go to the mountain, or take it to the mountain, means to act as if you lived in the mountains. Some people take this literally to act like cavemen, others only as far as lumberjacks. Can be incorporated with getting back to nature.
It originated with a boys' cross country team camping in the woods. They got lost on their run and one of them had to use a toilet. Because they were out in the open and didn't want to smell it while they were sleeping later, the team told him to take it to the mountain (pointing uphill to the top of the mountain they were on). So he did.
Real men don't camp in RVs, light campfires with matches, or use toilet paper. Real men go to the mountain.