The Bounce Rate for a specific page shows the number of times when this page was the only page viewed during a Bounce Visit divided by the number of times when this page was an entry page.
The Bounce Rate for a group of pages shows the number of times when the pages from this group were the only pages viewed during a Bounce Visit, divided by the number of times when these pages were entry pages.
The Bounce Rate for the whole site shows the percent of Bounce Visits out of the Total visits.
The Bounce Rate for a specific referrer shows the number of visits from this referrer where the site was exited after viewing a single page, divided by the total number of visits generated by this referrer.