The Yin-Yang symbol is widely recognized, though often people (especially Westerners) don't know it's true meaning. In the two days of having the symbol on my car, one person commented that it was related to hippie culture, and another thought that it meant "good and evil." In asking the question of its true meaning, of course there are many interpretations.  I always thought of it as balance, and more specifically that within the light there is always some dark, and within the dark there is always some light. Here’s what Wikipedia states:

In Chinese philosophy, the concept of yin yang ([yin – simplified Chinese: 阴; traditional Chinese: 陰; pinyin: yīn] [yang - simplified Chinese: 阳; traditional Chinese: 陽; pinyin: yáng] often referred to in the west as yin and yang) is used to describe how polar or seemingly contrary forces are interconnected and interdependent in the natural world, and how they give rise to each other in turn. The concept lies at the heart of many branches of classical Chinese science and philosophy, as well as being a primary guideline of traditional Chinese medicine,[1] and a central principle of different forms of Chinese martial arts and exercise, such as baguazhang, taijiquan (tai chi), and qigong (Chi Kung) and of I Ching divination. Many natural dualities — e.g. dark and light, female and male, low and high, cold and hot — are thought of as manifestations of yin and yang (respectively).

Yin yang are complementary opposites within a greater whole. Everything has both yin and yang aspects, although yin or yang elements may manifest more strongly in different objects or at different times. Yin yang constantly interacts, never existing in absolute stasis. The concept of yin and yang is often symbolized by various forms of the Taijitu symbol, for which it is probably best known in western cultures.

There is a perception (especially in the West) that yin and yang correspond to good and evil. However, Taoist philosophy generally discounts good/bad distinctions as superficial labels, preferring to focus on the idea of balance. The idea that yin and yang has a moral dimension originated in the Confucian school (most notably Dong Zhongshu) around the second century BCE.[2]
 

As Wikipedia states, the “taijitu” is the actual name of the black and white fish-like symbol people often recognize and refer to as yin yang in Western cultures. Here is what the site reads on the taijitu:

Taijitu (Traditional Chinese: 太極圖; Simplified Chinese: 太极图; Wade-Giles: T'ai Chi T'u; Pinyin: tài jí tú; Rough English translation: “diagram of ultimate power”) is a term which refers to a Chinese symbol for the concept of yin and yang (Taiji). The taijitu consists of a symmetrical pattern inside a circle. One common pattern has an S-shaped line that divides the circle into two equal parts of different colors. The pattern may have one or more big dots. The classic Taoist taijitu, for example, is black and white with a black dot upon the white background, and a white dot upon the black background.

Furthermore, the concept of yin yang plays a part in many healing practices. If you are interested in a summary entitled “Yin Yang in Medical Theory,” this is the first paragraph from the site (http://fly.cc.fer.hr/~shlede/ying/yang.html):

“The concepts of Yin and Yang and the Five Agents provided the intellectual framework of much of Chinese scientific thinking especially in fields like biology and medicine. The organs of the body were seen to be interrelated in the same sorts of ways as other natural phenomena, and best understood by looking for correlations and correspondences. Illness was seen as a disturbance in the balance of Yin and Yang or the Five Agents caused by emotions, heat or cold, or other influences. Therapy thus depended on accurate diagnosis of the source of the imbalance.”

After reading all of this, I found it particularly interesting that I reversed the image I drew on my car from the most commonly used taijitu, though it doesn't seem to matter because there are so many versions. There is also something pleasing about the black portion of the symbol being the negative, empty space on my rear window to look through.