How does analog work?

There are many types of analog systems, but the one thing that they all have in common is that a signal voltage represents some physical quantity.

One of the simplest examples might be a guitar and amplifier: the electromagnetic pickup on the guitar produces a voltage that represents the motion of the strings. The amplifier first increases the magnitude of that signal, and then its power. The electrical signal is then used to drive an electromagnetic loudspeaker which changes the signal back into physical vibrations. At each stage, the variation in signal voltage is analogous to a physical vibration.

This is in contrast to a digital system, in which a continuous sequence of binary numbers, rather than voltages, represent some changing physical quantity.