I remember reading about this on http://psadmin.wordpress.com/ blog. short and to the point.

Query Security Record Definitions:

You implement row-level security by having Query search for data using a query security record definition. The query security record definition adds a security check to the search.
Query security record definitions serve the same purpose as search record definitions do for panels. Just as a panel’s search record definition determines what data the user can display in the panel, the query security record definition determines what data the user can display with Query.
To get Query to retrieve data by joining a security record definition to the base table, you specify the appropriate Query Security Record when you create the base table’s record definition.