What Is Bounce Rate

Your bounce rate refers to how many of your users leave your site after visiting a page, without click through to visit other pages. A high bounce rate means that your content isn't relevant to the people viewing your site.

If you've got a high bounce rate then that means you either need to change your site copy (so that it reflects what your audience is searching for), or change your traffic source so you get more targeted people viewing your site.

Resources: 

Google Analytics for Startups

New SEO Metrics You Need to Know About