An amplifier (in the context of Folk) is any device which takes an acoustic signal (a sound) and reproduces it louder.
A gramophone horn and a Guitar's body are both examples of passive amplifiers. A passive amplifier is one where no energy is required to make the amplifier work.
Mostly, when we talk of amplifiers, we mean electronic amplifiers which require that a sound is converted to an electrical signal, fed into the amplifier which then creates a copy of that signal many times larger before being output.
Input devices tend to be Pick Ups for Instruments and Microphones for Voices and Acoustic Instruments. Output tends to be a Loudspeaker or Headphones.
The Amplifier is most often employed in a Public Address System.