The number of cycles passing a stationary point in one second is called the frequency of the sound. The unit expressing frequency is cycles per second, or hertz (named after German physicist Heinrich Hertz). One hertz (abbreviated Hz) is one cycle per second. 1000 Hz is 1000 cycles per second. The frequency of sound is actually the pitch of the sound. A high frequency sound has a high pitch, and a low frequency sound has a low pitch.
Here is a sine wave with a frequency of 100 Hz. (tone here)
This is 1000 Hz. (tone here)
And this is 8,000 Hz. (tone here)
The normal human ear can hear frequencies as low as 20 Hertz and as high as 20,000 Hertz. Many animals can hear higher or lower frequency sounds. Dogs, for example, can hear frequencies above 20,000 Hz. This is why dogs can hear a high-frequency dog whistle and we cannot. Humans can sometimes feel acoustical energy with frequencies below 20 Hertz as a vibration, without hearing the sound.