Physically, as you probably know, audio is a vibration. Typically, we’re
talking about vibrations of air between approximitely 20Hz and 20,000Hz.
That means the air is moving back and forth 20 to 20,000 times per
second.
If you measure that vibration and convert it to an electrical signal
(say, using a microphone), you’ll get an electrical signal with the
voltage varying in the same waveform as the sound. In our pure-tone
hypothetical, that waveform will match that of the sine function.
Now, we have an analogue signal, the voltage. Still not digital. But, we
know this voltage varies between (for example) -1V and +1V. We can, of
course, attach a volt meter to the wires and read the voltage.
Arbitrarily, we’ll change the scale on our volt meter. We’ll multiple
the volts by 32767. It now calls -1V -32767 and +1V 32767. Oh, and
it’ll round to the nearest integer.
Now, we hook our volt meter to a computer, and instruct the computer to
read the meter 44,100 times per second. Add a second volt meter (for the
other stereo channel), and we now have the data that goes on an audio
CD.
This format is called stereo 44,100 Hz, 16-bit linear PCM. And it really is
just a bunch of voltage measurements.