Electronics are often times based in the "binary"
language, meaning that everything is presented by
0's and 1's. It is the easiest scenario because
you only need to find out of something is "on" or
"off." CD's are composed of many concentric
grooves, much like the old vinyl records. Each
groove of a CD is like a long trench. However,
the trench is not uniformly deep, but rather there
are two different heights. Imagine the bottom of
the trench being a race track with a set of
irregularly spaced hurdles. Now, a laser shines
into the trench, hits the bottom, reflects back
out, and then hits a detector that records the
event. Depending on if the laser hits the bottom
of the trench or hits the top of a hurdle, a
different signal is observed by the detector.
Since we have only two levels, there are two
different output signals. One signal is assigned
a value of "0" and the other a value of "1" - this
is how binary information is stored on a disc!
The binary signal is then decoded by the CD player
to be turned into the music that you hear.