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

They are usually written by 3rd parties, though a few come as part of a standard Ardour install. The sources for plugins are many and varied; see here for 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.