The bounce rate is the percentage of visitors who leave a web site after viewing a single page.
This is widely regarded as a bad thing because it may mean that visitors did not find what they were looking for and "bounced" away to another site. They left almost immediately.
However, this is a broken metric in many cases. For example if you were looking for opening times of a retailer in your city and you found that info right on the homepage of the retailer, you will technically leave the site after a single page view. However it does not mean you did not find what you were looking for.
Furthermore you may have visited some competing sites in a row, "bouncing" from on e to the next, still finding the exact information (closing times) on each of them in a single page view each.