Audio synthesizers are apps or plug-ins that user can use for digital audio generation or enhancement.
Audio generators can not only create sounds, but they can also generate waveforms that can be subsequently combined and/or processed.
These types of tasks have been originally performed using specialized hardware devices, but software based synthesizers' advantage is that they can be run on almost any computer with a simple sound card.
Synthesizers can be standalone tools or they can function as simple plug-ins for other apps (audio hosts), and are meant to help their users to easily enhance the functionality these 3rd party applications.