From Bulbapedia, the community-driven Pokémon encyclopedia.
The Sky Prism hovering atop the Sky Pavilion
refracting the four rainbows of the PokéPark.
The Sky Prism is an ancient stone in PokéPark Wii: Pikachu's Adventure. It is located high above the PokéPark on top of the Sky Pavilion. It represents and protects the friendship of all Pokémon. In ancient times, people that used to reside in the PokéPark constructed a temple to worship the Sky Prism. The people have since left the PokéPark, but the Pokémon that live there still tend to the temple.
One day conflict erupts in the PokéPark and Pokémon begin to quarrel. This causes the Sky Prism to shatter into 15 pieces, 14 of those pieces fall to the PokéPark below where the Attraction hosts find them. With the Sky Prism in pieces, the Sky Pavilion begins to fall towards the PokéPark, a collision that could destroy everything. Recognizing the impending doom, the guardian of the Sky Prism, Mew, pays a visit to a small village where he meets four friends; a Pikachu, a Piplup, a Charmander, and a Chikorita. Recognizing the deep bond the four Pokémon share, Mew sends them to the PokéPark in the hopes that together, they can restore the Sky Prism. When the Pikachu arrives, Mew visits it in a dream informing the Pikachu of the situation at hand, and gives it the task of finding the 14 pieces of the Sky Prism, thus restoring the friendships within the PokéPark. Mew then sends telepathic messages to the various Attraction hosts, telling them to give their Sky Prisms to the Pikachu. With every Sky Prism piece obtained, the bonds between Pokémon begin to grow and once the Sky Prism is restored, it creates four rainbows that arc over the entire PokéPark, representing the eternal bond between Pokémon.