As hinted at by the word "dual" within it, duality refers to having two parts, often with opposite meanings, like the duality of good and evil.
Everything has a Dual Nature
Everywhere we look we see duality or opposites - male and female, good and evil, hot and cold. One is dependent on the other and they cannot exist without each other. For example we would never know of the concept of hot if cold didn't also exist. The entire concept of temperature wouldn't exist either - we wouldn't need furnaces, we couldn't bake a cake, and we wouldn't know know what a hot shower is.
Yin and Yang
The Chinese described duality through the concept of Yin and Yang
In Chinese philosophy, yin and yang describe how seemingly opposite or contrary forces may actually be complementary, interconnected, and interdependent in the natural world, and how they may give rise to each other as they interrelate to one another. - Wikipedia
The Chinese martial art form, Tai Chi (taiji), is the study of how the two forces of yin and yang are manifested in the body. Yang is an outward projecting force such as a punch. Yin is an inward pulling force that is used to draw an opponent toward you. You can only develop yang if you also develop and are aware of yin. The stronger the yang is the stronger the yin will be.
Everything we know can be divided into having two aspects, otherwise we wouldn't know about it. My favorite way to view yin and yang is by thinking of the Sun and Earth. These two bodies demonstrate yin and yang accurately in almost every way. For example yin is female and in comparison to the Sun, the Earth is also female. We call earth our Mother Earth. Yang is male and so is the Sun. In this case we think of our Father in Heaven. Likewise the Sun is hot, bright, expressive, and associated with fire - all yang properties. The earth is cool, dark, nurturing, and associated with water - all yin properties.
The table below shows how the Chinese have divided thing into either Yin or Yang.
|Sex||Male - Father in Heaven||Female - Mother Earth|
|Emotion||Excited, Anger||Calm, Love|