This video guide from GreatScott! explains how to build a tiny Class A amplifier for your smartphone to boost headphone volume.
As the name suggests, an amplifier is a device that increases the amplitude (the power) of an electrical signal. Amplifiers can be used for any kind of electrical signal, but audio amplifiers are the most well-known. As an example, the audio output from an electric guitar is very faint, because that signal is produced entirely passively by the electromagnetic pickups. To push that up to a respectable volume, an audio amplifier is necessary. If your headphone volume coming from your smartphone is too quiet, GreatScott! explains how you do the same thing by building a tiny amplifier for headphones.