Amplitude usually refers to the measure of a wave, or phenomenon transmitted by waves, like sound, electromagnetic radiation (also light); high/low are correct; the height of the wave as a function of time or space, is its measure; the (time or space) average of the square of this measure is the intensity, related to the energy content.

Usually it means how loud or quiet something is. It's a posh word for 'volume.' It comes from the word 'ample,' which means 'plenty.' It is correct to say high or low amplitude.