Understand the nature of a frequency. When you open your mouth to speak, the sound you make comes from a vibration in your vocal chords that produce energy to make the sound. The energy comes out of your mouth (or off of a guitar string) in the form of an evenly spaced wave of air molecules. This energy wave then travels its way into your ear canal and is perceived.
Understand a sound frequencies tone. The tone of a frequency varies by the spacing of the wave in the frequency. To understand this better, imagine holding a coil. If you stretch the coil out, you increase the spaces between each high point in the coil. In a wave frequency, this would produce a low pitched sound, such as a deeply voiced male. Push the coil together and you decrease the distance between each high point, or decrease the distance between each wavelength, resulting in a higher pitched sound like that of a female voice.
Learn the ranges of sound frequencies and the limits of human hearing perception. Tones range from very high to very low, but only certain ranges are detectable by humans, meaning that frequencies too high for our hearing range, or with wavelengths too close together, will produce no audible noise to us at all. Dogs have a higher frequency range than humans, which is why they can hear the high pitched shrill of dog whistles almost inaudible to humans. Elephants, on the other hand, can hear lower pitches than humans because their large ears can detect shorter wavelength frequencies. Where they might hear a low sound, we would only hear silence.