I don't like the pictures or explainations I found in a quick search, but that picture will do... Along with this quickie explanation.
I - Top left, things you know about yourself, that others also know about you.
II - Top right, things you don't know about yourself but others know about you. This pane is your blind spot (this is what I meant when I said it was your johari window)
III - Bottom left, things you know about yourself, but others don't know about you.
IV - Bottom right, things no one knows about you, including yourself.