I first Saw this rock when i started the game and had no clue what it was or what it was for. All it said was "It's a challenge rock". Any answer would be much appreciated. :)
Inside the outer area, there is a massive part of the forest that leads up to a rock called Challenger Rock. This rock, surrounded by Fighting trainers will give you a Star Piece item every day when you take a Fighting-type Pokémon to it.