Unlock the desktop and you'll be able to add and remove Plasma™ widgets from panel or desktop. You add Plasma™ widgets by simply dragging them where you want them. click on an widget to remove it.
The button allows you to add widgets you've previously downloaded and download new Plasma™ widgets. Currently it supports native Plasmagik packages and some Mac® OS X dashboard widgets. Widgets you install this way can then be accessed just like regular, preinstalled widgets.