Bounce rate is the percentage of users that visit just one page of a website before exiting.
Bounce rate is an important metric for an SEO to measure. It's an indication of the quality of user experience, which includes both content on the page and navigation from the page. A common cause of a high bounce rate is an unclear internal linking structure that discourages users from navigating to another relevant page on the same domain.
Articles targeting long tail terms, such as a blog post, will tend to have a higher bounce rate as users with long tail queries tend to be after a quick answer. You can attempt to reduce this by offering links to similar articles that users on this page will be interested in and using internal links within the body copy.
Pages targeting more transactional queries, such as a homepage or product page, should have a much lower bounce rate. If the bounce rate of your homepage is higher than 50 or 60%, you should rethink the user journey and navigational links from that page and perhaps review the keywords that you are targeting on that page.
Google offers its own advice on bounce rates, which you should take in to account as it's likely that Google take a version of bounce rates into account in their ranking algorithm. While Google don't have a reliable method of knowing whether or not a user bounced they are probably measuring how long it takes between searches with long clicks and short clicks.
A short click is where a user performs a search, visits a website briefly, then returns to search again. It indicates to Google that the search result on that particular page was unsatisfactory.
If a user takes a long time to return to search after the initial query, this indicates to Google that the users search query has been satisfied and the information on that page is of a high quality.