Want to make a pretty sound? Learn to control sound wave vibrations.
Sound waves move at a specific frequency. A wave's frequency basically just indicates how rapidly or slowly a medium vibrates as a sound wave passes through it. Scientists use Hertz (Hz) units to refer to frequency; a single vibration per second is 1 Hz.
Ears are constructed to pick up on fluctuations in frequency, because a sound's pressure waves affect the eardrum. Humans often refer various frequencies with the term pitch. A high-frequency sound is higher in pitch; lower-frequency sounds have lower pitch.
Play specific frequencies simultaneously and you'll create lovely sound. That's especially true when a second sound wave has, for example, twice the frequency of the first. We denote this scenario with a frequency ratio of 2:1, which is also called an octave.
With a musical instrument, you can create all sorts of different frequency ratios, such 4:3 or 3:2. Some of them sound especially nice to the human ear, and we put them to use in songs. A lot of music, then, is ultimately a blend of sound waves with whole number ratios between their frequencies.