The exit rate is the percentage of visits leaving the website from a particular page or content area.
The exit rate is calculated as the number of exits divided by the total number of visits.
The number is only interesting if it is calculated for a particular page or content areas since it otherwise equals 100%.
Notice that if you have more than one websites, a visit could exit on another of your websites. In this case the last page view on the website you are analyzing will still be considered the exit page.