To know a language- What does that mean?????

Whatever else people may do when they come together- whether they play, fight, make love or make automobiles- they talk. We live in a world of words. We talk to our friends, our associates, our wives or husbands, our lovers, our teachers, our parents. We talk to bus drivers and total strangers. We talk face-to-face and over the telephone. And everyone responds with more talk. Television and radio further swell this torrent of words. As a result, hardly a moment of our waking lives is free from words, and even in our dreams we talk and are talked to. We also talk when there is no one to answer. Some of us talk aloud in our sleep. We talk to our pets and sometimes to ourselves. And we are the only animals that do this- that talk.
The possession of language, more than any other attribute, distinguishes human from other animals. To understand our humanity one must understand the language that makes us human. According to the philosophy expressed in the myths and religions of many peoples, it is language that is the source of life and power. To some people in Africa, a newborn child is a kuntu a "thing", not yet a  muntu, a "person". Only by the act of learning languages does the child become a human being. Thus according to this tradition, we all become "human" because we all know at least one language. But what does it mean to "know" a language?
                                                     Victoria Fromky, Robert R and Nina Hyams, An Introduction to Language (abridged and adapted)