In short
The amplitude of a wave or a vibration is the largest displacement of a particle from its rest position. It tells you how much energy the wave carries.
Measure it from the rest position to the crest, not from the trough to the crest. A bigger amplitude means a louder sound and brighter light. Amplitude is not the same as frequency or wavelength: amplitude is how tall the wave is, wavelength is how long one wave is, and frequency is how often waves pass.
A pendulum bob swings between to the left and to the right of its lowest point. The amplitude is , not .
Also searched as: amplitude and frequency of a wave, difference between amplitude and frequency.