Yin and Yang are two exactly opposite things but properly dependable on each other. One exists only because of the existence of the other. They show the balance and harmony of our life.
Example of Yin Yang :-
Yin
Yang
Day
Night
Summer
Winter
Cold
Hot
Odd
Even
Male
Female
From above example we can understand one exist only if the other exists. We can not imagine about Night if there was no Day. We can not realize Winters if there is no Summer. In the same way we can feel the hot only if we know about cold. Happiness we realize only if we know about sorrow.