Yin Yang is a Chinese symbol representing the perfect balance. Yin Yang is a modern way of originally saying Ying Yang.
In many ways, it means that there is good in bad, and bad in good. Yin being the 'bad' or 'dark' one, and Yang being the 'good' or 'light' one. In Yin there is Yang, and in Yang there is Yin. They balance each other out.
In other ways, it also shows how opposites attract.