Here are some examples that may make is easier to understand;
Yin: Moon, Female, Water, Passive, Shade
Yang: Sun, Male, Fire, Aggressive, Sunlight
This concept can then be expanded further by adding that they are interdependent, that is they cannot exist without each other. We cannot have night without day, so as night becomes day, there is Yin within Yang!
In Traditional Chinese Medicine we use Yin and Yang as a way of describing energy, emotions and functions which make our bodies and minds tick. Keeping a balance of Yin & Yang within our energy systems is the key to health.
There are four conditions of imbalance;
Excess of Yin
Excess of Yang
Deficiency of Yin
Deficiency of Yang
A good example is something we see quite often, Kidney Yin Deficiency, the symptoms of this condition can be dizziness, vertigo, weak lower back, weak knee joints, dry skin and night sweats etc. So in this case we use acupuncture to address this imbalance by replenishing the deficient Yin energy.