They're the two stars of Pokemon White and Black (just switch the colors around to get which applies to which game): Zekrom and Reshiram. Zekrom represents ideals, or how we believe the world should work, and Reshiram represents truth, or how the world is. These two themes permeate the games and drive the story. I don't know about White, but Black's central conflict arises from the blind pursuit of an ideal world in which Pokemon can be free of the suffering one trainer, N, believes their trainers put them through without understanding the truth of the bonds of friendship between many trainers and their Pokemon.