Lao Tse begins his Dao De Jing with these stanzas. The meaning is both simple and complex. The first two say we can experience perfection, even know it, but not describe or teach it. (It is always so with mystical quantities and qualities. It's what makes them mysterious and, therefore, mystical.)

Recognising the relevance of the Yin & Yang symbol to these stanzas is a little more subtle. The simplest interpretation is the familiar saying "you cannot have light without darkness." A Daoist might say that darkness and lightness create each other. That is the balance visually described with the Daoist symbol of yin and yang.