The Yin and Yang is a very ancient philosophical concept, basically it consists in that everything which happens in the world has 2 sides, one is the active one and other the passive one. Typically the Yin and Yang is represented with this sign:
Before this sign was invented, it was called the receptive (Yin, represented by this sign: ) and the creative (Yang ).
Regarding natal charts, what you have to keep in mind is that Yang represent the active, what takes the initiative, that is clear on the surface and easy to spot.
On the other hand, the Yin represent the inside, people who don't talk a lot and may be a little shy, also is related to spirituality and sensitiveness.
Hopefully one day I will talk more and better about this!