Well, in PC terms it means to "stretch" the window in which an application appears so that it completely fills the screen from one edge to the other.  A window that is maximized cannot be moved or resized. 

To maximize, click the button immediately to the left of the close ("X") button in the upper right corner of the window.  (This is a toggle button; click it again to remove the maximize option and return to a normal window that can be moved or sized freely.)