Plugins are bits of software that get loaded by Ardour in order to:

Ardour does not come with any built-in signal processors of its own (other than volume faders) but does ship with a small group of plugins starting at Ardour 5.0. The shipped plugins are listed as authored by "Ardour Team" and named with "a-" as the start of the name (Like a-EQ) or Authored by "Ardour LUA Task Force" in which case they are example (but still useful) LUA scripts. The included plugins are LV2 or LUA scripts and use Ardour's generic GUI. They work on all supported platforms so that projects started on one platform will sound the same on another platform if they use just these plugins or other plugins that are cross platform. They are written by 3rd parties, though we do provide some information on how to get them.

Ardour supports a variety of different plugin standards:

LADSPA
An early, simple, lightweight plugin API, audio effects only, plugins have no editors/GUI of their own (Ardour provides one, however).
LV2
An extensible, full-featured plugin API, audio and MIDI, plugins can provide their own GUIs but may use the one Ardour provides instead.
AU
OS X only, full featured, audio and MIDI, plugins can provide their own GUI
VST
Plugins using Steinberg's VST plugin standard. Varies by platform:
on Linux
(native) Linux VST plugins fully supported (VST2.4)
on Windows
(native) Windows VST plugins fully supported (VST2.4)
on OS X
(native) macOS VST plugins fully supported (VST2.4) since Ardour 5.5
Windows VST Plugins on Linux
VST plugins for Windows, but being used on Linux. Not supported by normal builds of Ardour. Read more…

Adding/Removing/Copying Plugins

Within Ardour, plugins are just another type of Processor and so the techniques for adding/removing/copying/moving processors apply to plugins as well. These techniques are covered on the Processor Box page.