Sound

Sound travels through the air as a wave: the air pressure rises and falls, and your ear turns those changes into what you hear. If you drew a sound on a graph with time going across and the wave's height going up and down, you would get a smooth curve. Sound in the real world is analogue, which means it changes continuously: the wave has a value at every instant, with no gaps and no steps between one value and the next.

At any moment you pick, the wave is some distance above or below its middle resting line. That distance is the amplitude at that moment. A tall peak is a large amplitude; a point near the resting line is a small amplitude. Amplitude is not fixed for a whole sound: it changes as the sound changes. Picture a single drum hit. The moment the stick lands, the wave shoots up to a tall peak, and as the sound dies away the peaks shrink. A quiet moment has a small amplitude; a loud moment has a large amplitude.

Amplitude at a moment is not the same as how quickly the wave repeats, which is what sets the pitch you hear. A high-pitched whistle repeats quickly but can still be quiet or loud.