Plug-Ins are scripts programmed in Python that modify existing features or add new features to QuArK. They are mainly related to the map editor, which is itself programmed mostly in Python. Plugins as used in QuArK are actually quite different from conventional plugins as found GtkRadiant etc., so here's a bit of background to forestall confusion.

QuArK was originally written entirely in Delphi, but halfway through writing QuArK 5, Armin Rigo discovered Python and decided that it would be better than straight Delphi for programming the QuArK GUI. The architecture he came up with (I don't know what its antecedents are) was that each aspect of the interface would be defined by a plugin file, sitting in the subdirectory plugins/, which would be able to call on both a set of facilities provided by Delphi, and also a collection of facilities provided in Python by files in the quarkpy/. So when map or model editor is started, all the corresponding plugins are loaded, and their functionality enabled.

Probably the best way to get a feel for plugins is to first work thru a tutorial, then look at some existing examples. There is an archive here, plugin_examples.zip, of working examples for this tutorial (and others) in the 'zips' sub-folder of your QuArK-help folder on your hard drive.