According to the Webster's Dictionary, sound is:
a: a particular auditory impression; the sensation perceived by the sense of hearing;
b: mechanical radiant energy that is transmitted by longitudinal pressure waves in a material medium (as air):
c: is the objective cause of hearing

For the purposes of this course, we will use the second definition - energy transmitted by waves through the air.
The graphic at the right illustrates this wave motion.
At the center is a tuning fork that is vibrating. The surrounding air molecules (represented by + in the graphic) transmit this vibration. When this energy reaches our ears, we perceive this as sound.

Frequency

We can measure these vibrations by determining how many times a sound wave moves back and forth in 1 second.

1 Hertz = 1 vibration per second

Humans cannot hear this slow vibration; the slowest (lowest) wave perceptible by human ears is about 20 vibrations per second (20 Hertz or 20 Hz).
The lowest note on a piano is about 30 Hz; to sound the highest note on the piano, the wire vibrates about 4200 times a second (4200 Hz).

On some pipe organs the lowest pipes (the biggest ones) can produce vibrations of slower than 10 vibrations per second (10 Hz). Humans cannot hear these tones (but you can feel them).

Most humans can hear vibrations between 20 - 20,000 times a second (20 - 20,000 Hz)
Dog whistles are usually in the 16,000 to 22,000 Hz range