This means that the counterpart entries will not be owned by each entry, but globally for the given journal and period. You will then have a credit line and a debit line centralized for each entry in one of these journals, meaning that both credit and debit appear on the same line.

Check some of the docs, previous questions or blog posts on Opening Entries - this is when they are used.

http://mohsinpage.wordpress.com/2010/08/04/generate-new-fiscal-year-opening-entries-year-based-balance-forwarding-in-openerp/