The Fundamental Frequency is the lowest frequency of a sound wave. When you pluck a guitar string, it causes the string to vibrate and produce waves with different frequencies. Our brain interprets those multiple frequencies as a specific pitch. In reality, that one sound we hear is composed of multiple frequencies, the lowest of which is the Fundamental Frequency. A lower pitch, like a man's voice, would have a lower Fundamental Frequency than a higher pitch, such as a child's voice.