The bounce rate is a marketing metric that allow you to judge the quality of your traffic. It's the the percentage of visits of visitors to visit only one page and then leave. For example, if you have 100 visitors on your website, 20 of them visit just one page and quit, the rest of them visit at least 2 pages, your bounce rate will be 20%.
It could be interesting to check it depending of the source of your traffic. If the bounce rate a partner provide is high, maybe you should cancel this partnership? On the opposite, if the bounce rate for a special keyword for visits from Google is really low, maybe you should focus on this keyword!
The goal of a content website is to provide the answer to a visitor, and then having him to click on an ad. A bounce rate of 60% is still pretty OK.
Visitors on ecommerce websites should visit many pages before buying a product. The lowest bounce rate is the better. One of less than 20% is pretty good.
People looking for local shop are looking for what the shop is doing (so they will visit many page), or just a phone number or menu. A bounce rate of 40% is correct.