When a violinist plays a note on a violin string, the string starts to vibrate very fast. This vibration makes the air vibrate and the sound waves travel to our ear so that we can hear it. If the note were absolutely pure the string would move in a perfect sinusoidal shape and produce only one frequency, but instead there are multiple frequencies being produced at the same time along with the main one. The note played on the violin string makes the string vibrate in a very complicated way. There is the basic note (the fundamental), but added to that are lots of other little notes that all add up to a sound in a special way that tells us that it is a violin playing and not a clarinet or a human voice.