It is just like efficiency, that amount of time use spend on your site. Many say that higher is your bounce rate lesser is the possibility of user interaction and hence less possibility of sponsored links getting clicked, and that mean less sponsors !
The ideal bounce rate percentage is as low as possible... ie. 0%
Bounce rate is the percentage of users who leave your site upon first visiting it. A low bounce rate is not only an important sign that visitors are engaged with your website... but it is also an important signal to search engines like Google.
Google tracks clicks from their SERPs (search engine result pages), and if they see that someone visits your site and then quickly "bounces" back to the search results, they will take that as a sign that your site is of low quality or not useful, and may adjust your ranking *shudder* downward.
Bounce rate, pages per visit, and time per visit are all important indicators of how engaging your site is.