From the ALSA wiki
A synthesizer is a hardware capability of a soundcard which can be used to generate an audio stream from software rather than from recordings. To emulate a real musical instrument, sample notes are recorded and stored in a wavetable. The synthesizer steps through the wavetable reading out and playing back the sample values. Altering the step size when reading the wavetable allows the synthesis of notes with different frequencies.
Retrieved from "http://alsa.opensrc.org/Synthesizer"