From ComputerCraft Wiki
|A function available to window-based terminal objects created via the window API, which re-renders everything that has been rendered via the window in concern (assuming it is visible). This includes content rendered via child windows.
Generally this is handy if one window has written content over the top of the top of another (of which it is not a child), and you wish to bring the content of the covered window back to the fore-front of your display.You need not call this function when simply making a window visible or when moving / resizing it; they call it automatically. Redrawing a window also restores its cursor.