Transistors are at the basis of amplification. With a transistor you can take a smaller voltage and increase it to a larger and workable voltage. This is important if you are trying to take something like a weak audio signal and make it louder. Many amps do multiple stages of this so that the noise floor doesn't carry into the final signal. Transistors are what allow this amplification process to occur.

There are two kinds of transistors, a bipolar transistor and a field effect transistor. We mostly focused on the bipolar transistor. The bipolar transistor has three terminals, the emitter, base, and collector. By turning the base on and off you can actually use the transistor as a switch.

By varying the resistance you can control how much voltage is traveling across the base and emitter. If the voltage reaches a certain level it will cause the transistor to begin "clipping" the signal. This is where the top and bottom of the output signal is flattened and causes the tone of the sound to become distorted. Whether distortion is a desired tone or not is dependent on the style of music.

Below is a video of a distortion circuit that I made: