As a guitar player, this hurt me deeply
Hermione, this is the difference:
Basically, a clean sound comes from pushing less power through the amplifier tubes, which causes the sound to break up less, and sound, well, cleaner. A distorted sound comes when you push more power through the tubes, which causes the sound to break up in uneven ways. You can control the level of breakup on most modern amplifiers with a dedicated control for it, but older amplifiers had to have the volume turned up very loud for that kind of breakup to occur.