The amplifier itself is a device which increases the power of signal. An amplifier functions by taking power from a power supply, on the other side controlling the output to match the input signal shape, but with larger amplitude. In other words an amplifier modulates the output of the power supply based upon the properties of the input signal, and we can call it the opposite of an attenuator as an amplifier provides gain, an attenuator provides loss.
However amplifiers are the tiny components in hearing aids in order to make voices sound louder just like in radios where it boosts up the faraway signals and the devices in stereo equipment that drive the loudspeakers and the huge black boxes one plug into electric guitars to make them raise the roof. If you want to amplify a fluctuating signal, such as a radio or TV signal, the sound of someone's voice coming down a telephone line, or the input from a microphone in a hearing aid, for which you'd generally use a transistor-based amplifier. It has three wire connections called a base, an emitter, and a collector. When small input current between the base and the emitter is felt, you get a much larger output current flowing between the emitter and the. Before transistors were invented in 1947, much larger electronic amplifiers called vacuum tubes were used in such things as TVs and radios.