This statusbar tells you if you click the button or if you press any key (and which key).
Gdk.keyval_name(event.keyval) converts the key value event.keyval into a symbolic name. The names and corresponding key values can be found here, but for instance GDK_KEY_BackSpace becomes the string "BackSpace".
In line 17 the signal "clicked" is connected to the callback function button_clicked_cb() using widget.connect(signal, callback function). See for a more detailed explanation.
pop(context_id) removes the first message in the statusbar stack with the given context_id.
remove_all(context_id) removes all the messages in the statusbar stack with the given context_id.
remove(context_id, message_id) removes the message with the given message_id in the statusbar stack with the given context_id. The message_id is returned by push(context_id, "the message") when pushing the message on the statusbar.
In this sample we used the following:
GtkStatusbar
Gdk - Key Values