Amplitude is a property of a wave that gets the maximum amount of displacement of a particle of the medium from its rest position.
The amount of energy carried by a wave is related to its amplitude. A high energy wave is characterized by a high amplitude; a low energy one is characterized by low amplitude.
Amplitude is generally calculated by measuring the height of the wave from the resting position. For example, if one looks at a sound wave, the amplitude will measure the loudness of the sound. The energy of the wave also varies in direct proportion to the amplitude of the wave.