Sound is created from the vibrations of objects. These vibrations produce variations in the atmospheric pressure which propagate away from the objects in the form of sound waves. Our ears are capable of detecting incoming sound waves and converting them into nerve signals that our brain interprets as sound.
One way to visualize sound is to draw a graph of the variations in the atmospheric pressure at each moment in time. However, understanding how those graphs relate to what we hear can be extremely complex. For that reason, we usually start by studying the simplest type of wave, the sine wave.
The sine wave is interesting for educational purposes, because we can easily identify two of the main properties of sound from it: volume and pitch. Most audio libraries allow us to control both of these properties for any sounds that we play.
Volume: This property corresponds to how loud or quiet the sound is. It depends directly on the amplitude (or the height) of the sound wave, as measured on the vertical axis. The main unit of volume is the decibel (dB), but most audio libraries use a scale between zero (silence) and one (full volume).
Pitch: This property determines how high or low the sound is. It depends on the frequency of the sound wave, which is the number of times that it repeats every second. The unit of frequency is the hertz (Hz). Two things that you should know about frequency are that the human ear can only hear frequencies within the 20 Hz and 20,000 Hz range, and that most sounds that you hear are actually a combination of several different frequencies.
