Well the yin-yang symbol represents the complementary interaction of two entities, which together form a greater, stronger influence, whether it be in idea or nature. As for Kyurem, it is wuji, the lack of yin and yang, emptiness, which is why it has a lower stat total in its standard form.
Now on to the in-game legend. At one point, thousands of years ago, Zekrom, Reshiram and Kyurem were a single, highly powerful, dragon guiding the morals of an ancient kingdom. This kingdom entered a civil war following a falling out between the king's sons. The older sought truth, while the younger sought ideals. The dragon was divided by this conflict, to the point that it split in two, Reshiram, who desired truth, and Zekrom, who desired ideals. Kyurem was the leftover remnants. Truth and Ideals are at first glance separate, opposing entities, that in reality must compliment each other in order for one to make logical choices. Kyurem, being the remnants of the original entity, can absorb the former parts of its body (Zekrom and Reshiram) to regain it's former the strength (the very powerful Black Kyurem and White Kyurem). The civil war continued, and both sides were even matches. Eventually Reshiram and Zekrom became so weak from battle that they regressed into their respective stones, and the war ended. Thousands of years past until both of them were reawakened by N and the B/W Player Character, where the two fought, and team plasma was defeated. Two years later, N released his dragon to the B2/W2 player character.
Thanks for reading hope this helps