Frequency refers to audio frequencies. Frequencies are vibrations in the air. The number of vibrations per second is known as the frequency, measured in Hertz (1 Hz = 1 vibration per second). These vibrations cause the eardrum to vibrate, which enables us to hear sound.
The audible frequency range, for the average person at birth, is from 20Hz to 20000Hz (or 20 kHz). It is worth keeping in mind that the frequency range goes much higher and lower than human hearing. These sounds are called ultrasonic and infrasound.
This frequency range is also called the frequency spectrum.