Yin is one half of the primal forces that form the core of Chinese metaphysics (the other being Yang). These two energies, or forces, are believed to be present in all things; combined, yin and yang make up the energy, or Chi, of the universe.
Yin energy has many attributes attached to it; typically, Yin energy is represented as cold, static, soft, yielding, dark and hidden (among many other attributes). Yin is also considered a feminine sort of energy, though both genders have Yin and Yang energy in them.
It is worth noting that though Yin and Yang represent dark and light respectively, these energies have no connection to good or evil (as is common in some Western ideas); both are considered important energies in order to maintain universal balance.
The Pillar of Severity in the Tree of Life.