It's a raw 64 bits number that contains all information to render a guild icon. It's the number that is in the Character xml file under
s: (default) Small guild icon (32x32 pixels)
b: Big guild icon (64x64 pixels)
It's cached for ever. Since the icon is unique and never changes. For a specific icon, it'll always return the same image.
Returns a string that contains the url to display a guild icon.