Examining the circuit of an amplifier you’d find many electrical components, of which all play a role only they can fulfill.
Among these components, it is the transistor which plays the most crucial role.
Transistors consist of a base, collector and emitter.
The output current/voltage of the transistor is changed by the application of current/voltage at its input.
For example, a transistor with a base of beta 100, when applied 1 milliamp of current to the base (input) the current drawn by the collector (output) will change by 100 milliamps. Thus the current is changed by a factor of 100.