The content pane is the actual part of the window where you can put the main content of your window, as opposed to the menu bar and other "reserved" areas of the window.
This page has an illustration that will explain it a lot better than I can here.
http://java.sun.com/docs/books/tutorial/uiswing/components/toplevel.html
I've seen a Swing forum for Swing questions.
But people still seem to post all the questions in the Java forum.
Why did the forum designers create a Swing forum if everyone is still going to post Swing questions in the General Java forum?
Yet another mystery. Which one will be solved first, why people don't post in the correct forum or why a content pane exists?