The bounce rate is the number of single access pages divided by lands.  Bounce rate is the percentage of website visitors who arrive at an entry page on your web site, then leave without visiting any other pages or leave without going any deeper into the site. Bounce rate is measured as a percentage. Ideally you want your site pages to have a lower bounce rate.

From a sponsored search point of view, bounce rate is the number of times a visitor clicks on an ad, but leaves the landing page almost immediately.

Bounce rate is a useful metric that allows you to measure the quality of your content and the quality of your web pages. A high bounce rate generally indicates that site entrance pages are not relevant or useful to your visitors. It is an indication that the user was unable to find the information that they were looking for based on their query.