The Demo Actions plugin is a demo or “dummy” plugin. For a (self-)educational purpose.
It illustrates that: If you can write some program of any size in Java, you can write a plugin.
The menu item Buttons AB Demo opens the dockable window with two buttons on a panel.
The menu item Threads Demo opens the frame with the title Threads Demo Input Dialog. The fields of this dialog serve to get input to two runnables running in parallel threads. The runnables loop appending strings into one StringBuffer which is finally inserted in a new buffer of jEdit.
The menu item List Engines inserts in a new buffer the list of scripting engines that are "visible" from the plugin.