see http://www.w3.org/TR/xpath#path-abbrev
// is just an abbreviation for the descendant:: axis
Edit
To quote:
//para is short for /descendant-or-self::node()/child::para
That is, it refers to all para which are a child of the context node or any node descended from the context node. As far as I can tell that translates into any descendant para of the context node.