Frequency

Frequency in audio is the speed of the oscillating sound waves as they travel back and forth. It's measured in the unit, hertz, or cycles per second. When viewing frequency on a graph, it's described by the rate of change in amplitude, or essentially the number of cycles the waveform makes per second. For example, a tone of 100 Hz would push a speaker cone back and forth one hundred times in a second, because 100 Hz translates to 100 cycles per second. When frequency increases, the pitch of the sound wave also gets perceived as higher. In contrast, a lower frequency will create a lower pitch, which is a lot harder to hear with our eardrums, but can be felt because of the vibrations.
With age, we prematurely damage our hearing, and our ability to hear high frequencies is reduced, so it's important to protect one's hearing from noisy environments, so they can experience a larger field of sound and hear all types of frequencies.