Audio Units are plug-ins that can be used to process audio by applications that support Audio Units such as Triumph, Myriad, Ableton Live, Logic Pro, GarageBand and others. All Mac computers include a collection of Apple Audio Units, and many other developers distribute plug-ins in Audio Units format.
The Apple Audio Units plug-ins are built into macOS. Plug-ins from other developers appear as individual files on your drive. They may be found in two places:
/Library/Audio/Plug-Ins/Components — available to all users on the computer
/Users/yourhomedirectory/Library/Audio/Plug-Ins/Components — available only to the owner of the home folder