Amplifier

The amplifier controls the volume of your sound. The amplifier takes the set loudness of the synth and applies the volume envelope (as well as any other envelopes or instructions) to the resulting waveform. Amplifiers can also control the amount of presence an oscillator has among others within what is called a voice. Lots of hardware synthesizers for example provide a dial to set which oscillator (OSC1 or OSC2) has the more dominant volume (or if they're equal), and that's a way of configuring the voice of the synth.

Whenever a signal is amplified, its harmonic content will remain the same up until a certain point (total quietness or clipping). One of the ways you can drastically change the timbre (the shape and harmonics) of a sound is to amplify the signal beyond the limitations of the range of signals you can send to the speaker. This is called clipping.