Reshiram = yang
Zekrom = yin
Kyurem = wuji
Yang means light. Reshiram is yang, and its Pokedex entry says:
This legendary Pokémon can scorch the world with fire. It helps those who want to build a world of truth.
Yin means darkness. Zekrom is yin, and its Pokedex entry says:
This legendary Pokémon can scorch the world with lightning. It assists those who want to build an ideal world.
Originally, Reshiram, Zekrom and Kyurem originated from the same powerful Dragon Pokemon. However, it split because of twin heros who used it to create Isshu had different goals in life, one sought the truth and the other sought the ideal. When this Pokemon split, Reshiram sought the truth and Zekrom sought the ideal. Reshiram and Zekrom fought for each of the hero twins, but neither of them won, and the twins decided that neither truth nor ideal was more important than the other. Their descendants resumed the fight, destroyed Isshu with the two Dragons, which subsequently disappeared. In BW, you have to find the corresponding Dragon in order to stop N (who has called one of the Dragons to create his perfect world), and stop history from repeating.
Yin and yang is basically the theory of corresponding opposites; Reshiram is white (light) and Zekrom is black (dark). They are meant to be complementary forces, and not in opposition with one another, which is why when Reshiram and Zekrom fought they destroyed the whole Isshu region, as they were meant to work in harmony.
Also, people usually think that yang is good and yin is bad, but in Taoism it's simply meant to represent opposites, and a moral judgement is not attached to the yin/ yang philosophy, so that's why neither Reshiram or Zekrom were "better" than each other.
Wuji means without bounds (it literally means without a pole, i.e. 南極 means south pole, and 無極 means without a pole). The Pokedex entry for Kyurem in B2W2 says:
This legendary ice Pokémon waits for a hero to fill in the missing parts of its body with truth or ideals.
When the Dragon Pokemon split, Kyurem became what was left over as Reshiram became the truth and Zekrom became the ideal, and in B2W2, you can use the DNA splicers to splice Kyurem with either Reshiram or Zekrom, filling the void with either truth or ideals. It's also possibly why Ghetis was able to control Kyurem forcefully; while Reshiram and Zekrom gravitate towards a trainer that seeks what they seek, Kyurem is not the same, because it is lacking the driving force that Reshiram/ Zekrom have. This is Drayden's theory on Kyurem's origins, and it seems to be the most plausible; in Lacunosa Town the old story is that Kyurem came to the town, taking people away and eating them, which resulted in the great wall built around the town in order to stop Kyurem from returning.
The three Dragons are called the Tao trio because they represent elements in Taoist philosophy.
I studied History of Art and took a course in Chinese art, so I should know about this. x_O
Extra information (if you want):
Taoism - contains entries on Wuji, Yin and Yang
Bulbapedia's entry on Tao Trio
