Many site owners are clueless about what bounce rate actually is when they look at their analytics. Put simply, bounce rate is the percentage of visitors which come to your site, look at one page and then leave.
Bounce rate can vary hugely depending on the website you have. For example, my site is a content site so the average bounce rate is around 40-60%. You may think this is high but this is what all other related content sites also get. Because content sites mainly receive a lot of traffic from search results and referrals, the visitor will usually just find what they're looking for and then disappear. Social networking sites like Facebook will have a lot lower bounce rates because visitors usually look through a lot of content about their friends and things they're following.