If you use Google Analytics it is for sure that a percentage of the Bounces your site has are shown on the Panel page. That percentage is the Bounce Rate.
I always wondered what was the meaning of the Bounce Rate and what was the utility of that rate, so I started searching in the web to see what the meaning of Bounce Rate was and I found this definition:
"Bounce rate is a term used in web site traffic analysis. It essentially represents the percentage of initial visitors to a site who "bounce" away to a different site, rather than continue on to other pages within the same site."
But what is it for? The Bounce Rate for a web site is the number of web site visitors who visit only a single page of a web site per session divided by the total number of web site visits. So it is useful to see if your visitors find your site interesting. Also Bounce rates can be used to help determine the effectiveness or performance of an entry page.
An entry page with a low bounce rate means that the page effectively causes visitors to view more pages and continue on deeper into the web site.
I also found this video of Google.com analytics specialist Avinash Kaushik that explains the Bounce Rate: