The Texture Direction window can be used to indicate the direction of the predominant lay of the surface.  The Std parameter, Texture Direction, displayed in this window is the predominant direction of the surface.  This parameter is only meaningful if there is a predominant lay to the surface.  The Stdi parameter, Texture Direction Index, is the ratio of this predominant direction to rest of the direction spectrum.  It is an indication of how dominant the dominating direction is relative to the entire surface.  Notice in the images below how the surfaces with a very distinct direction have a lower value for Stdi.  A Stdi value larger than 0.5 is a pretty good indication that there is no "direction or lay" to the surface (isotropic surface).