An audio plug-in is 3rd party software program that can be added to a digital audio workstation (DAW) to extend its native library. These digital audio extensions are often used as effects, such as audio analyzers, reverb, delay, chorus, compression, and EQ, to correct or enhance sound recordings. They also come in the form of virtual instrument plugs, such as drum machines, modular synthesizers, samplers and etc.

For example, emulating outboard gear such as noise gates, compressors, brickwall limiters, and parametric equalizers means an end-user has capable tools at reasonable costs to make their music sound better assuming they've got the know-how to do music tings. However, it's not all about analog emulation. Modern tools provide end-users with numerous opportunities to achieve a professional contemporary commercial sound as long as they have enough processing power. Some plugs are notoriously CPU-intensive though.

Plug-in formats​

Popularly used plug-in formats include VST, VST3, AU, and AAX. All of these formats except Audio Units (AU) can run on both Microsoft Windows and Apple macOS platforms. This is because, like most applications made by Apple, you have to stay in a safe walled garden (or Apple's ecosystem to utilize them).

Furthermore, Avid Audio Extensions (AAX) can only be used in Pro Tools audio recording software. But with a VST (Virtual Studio Technology) plug-in there are no limitations. You can use them in whatever music production software (or VST host application) you want e.g., Ableton Live, PreSonus Studio One, Cubase and etc.