The parent_left and parent_right fields are an implementation of the Nested Set Model concept of relational databases, and not exclusive to stock_location.  There are a number of scenarios where they are used to define nested hierarchy: account_account, product_category, ir_ui_menu, etc.  The idea is that given any one record, the parent_left and parent_right fields can define all of the children, children of children, and so on found under that record, and in O(1) complexity.  SerpentCS has written up a much better explanation of this concept already: http://www.serpentcs.com/serpentcs-openerp-parent_left-parent_right-explained