Definition: Frequency is, quite basically, the universal unit of measurement of audio signals.
Frequency is measured based on the cycles per second of the signal. Higher frequencies move faster, and thus have more cycles per second (the upper limit of human hearing is widely regarded at 20kHz, or 20,000 cycles per second). Low frequencies are slower; 20Hz is considered the low end of human hearing, which means 20 cycles per second.
Frequency is measured based on the cycles per second of the signal. Higher frequencies move faster, and thus have more cycles per second (the upper limit of human hearing is widely regarded at 20kHz, or 20,000 cycles per second). Low frequencies are slower; 20Hz is considered the low end of human hearing, which means 20 cycles per second.

