Textbook chapter: 4 (though it's rather week on the subject).
Sound is a wave in the air. As any wave it has an amplitude and a frequency. The amplitude controls the loudness and the frequency controls the pitch.
People perceive higher frequency sounds easier than low frequency sounds. So even though the apmlitude is higher that doesn't mean necessarily that the sounds is louder - it also depends on the frequency.
Low frequency sound is also less directional, it's hard to figure out where exactly it's coming from.
Sound frequency is measured in Hertz (Hz). Humans can hear frequencies roughly between 20 Hz and 20k Hz.
Uncompressed Sound =
There are various ways of representing waves digitally. Usually the choice is to represent a wave digitally is as a sinus formula with some parameters. This doesn't work very well for sound waves because there are so many changes to the wave in each second.
The choice in an audio CD is to "sample" the wave repeatedly and record each sample as a number. An audio CD and a typical uncompressed digital audio file stores 44100 records for each second of audio.
The number of samples per second is also measured in Hz, so the 44100 above is typically represented as 44.1kHz. This measurement is not related to the frequency/pitch of the sound even though it's the same unit.
44.1kHz was chosen for raw audio because people cannot hear better than half that frequency, thus cannot tell that the reconstructed wave is different from the original. Some claim that they can distinguish a live performance from an Audio CD playback but even such claims are rare.