Mailing the story:

Whether for a word processor, an image editor, or a Web browser, plugins have become a common characteristic of modern application software design. An audio plugin functions generally in the same manner as any other kind of plugin: it extends the capabilities of the host application in some way. For example, a DAW (digital audio workstation) may have its own reverberation effect, but if the workstation supports plugins I can substitute a 3rd-party reverb plugin with preferable features and sound quality.