This is a term often used by Google to describe the number of times a single page is visited on a site. In otherwords, the visitor looks at a page on the site and then 'bounces' to another site.
Bounce rate is expressed as a percentage where the percentage number represents the number of visitors that viewed only one page on the site.
Every site owner should try to reduce the bounce rate to the smallest possible number - i.e. keep visitors on the site in order to increase the conversion rate for the site. To achieve this, there needs to be compelling calls to action that keep the user on the site and encourage a longer visit.