A rock star plugs his guitar into a amplifier. The amplifier

An amplifier (often loosely called an "amp") is an electromagnetic or electronic component that boosts an electric current. If you wear a hearing aid, you'll know it uses a microphone to pick up sounds from the world around you and convert them into a fluctuating electric current (a signal) that constantly changes in strength. A transistor-based amplifier takes the signal (the input) and boosts it many times before feeding it into a tiny loudspeaker placed inside your ear canal so you hear a much-magnified version of the original sounds (the output).