When something causes molecules of air to vibrate, your ear perceives this vibration as sound. For instance, when lightning strikes, the air around the flash becomes superheated and expands rapidly. This rapid expansion of molecules produces the change in pressure that eventually reaches your ears as thunder.
Sound is typically represented as an analog (continuous) waveform. The waveform describes the vibration of the air molecules. For example, the plucking of a guitar string not only visually resembles a waveform, it produces them as well.
The distance between the top (or bottom) of the waveform and its baseline is known as its amplitude. Amplitude indicates the volume of the sound. The points in a waveform with the greatest amplitude sound the loudest. Points in the waveform with little amplitude sound the quietest. A flat line in a waveform indicates silence.
All waveforms divide into periods. A period is the distance between two consecutive peaks in a waveform. The frequency of a waveform is determined by the number of periods that occur in one second. One period per second equals one hertz (Hz). One thousand periods per second equals one kilohertz (kHz).