The term amplitude is used to describe the volume of an audio signal and is related to how much energy is being transferred. For example, you would find a low amplitude when someone is whispering whereas someone shouting would produce a higher and stronger amplitude.
Sound waves are created by vibrating air molecules. You may have seen sound waves being represented in a wave-like graph, with peaks and troughs showing the outer limits of the sound. These peaks and troughs represent the maximum movement of the air molecules vibration.
Imagine a line running through the middle of the sound wave, equal distance between the peak of the sound wave (compression) and the bottom of the sound wave (rarefaction). The movement of air molecules from this line to either the peak or trough is called the amplitude.
So, in summary, the amplitude shows how far the air molecules vibrate from the middle line (equilibrium position) to the outer limits of the sound wave. The further the air molecules vibrate from the line, the louder and bigger the sound will be.
Synthesizers are able to recreate this natural process, the vibration is known in the synth world as oscillation. Essentially, an oscillator is capable of producing a pulsing electrical signal that creates basic waveforms known as sawtooth, triangle and square.
These three basic waveforms can then be modified and shaped into all sorts of different tones and pitches via the effects and filters options on the synthesizer, finally, it is amplified through speakers to reproduce a sound that our ears can understand.
The amplitude of the waveform (the volume of the audio signal being created by the oscillator and modified by the filters and effects) can be adjusted via a separate synth function known as a Voltage Controlled Amplifier (VCA), giving you full control of the output volume.