100 visitors come to your website and leave it after opening only one page = 100% Bounce Rate
100 visitors come to your website, 50 leave and 50 stay and open another page = 50% Bounce Rate
100 visitors come to your website, 20 leave and 80 stay and open another page = 20% Bounce Rate
High Bounce Rate = BAD
Low Bounce Rate = GOOD
It doesn't really affect the website much, but it shows how interested your visitors are in your website. If you have a high bounce rate, then try adding related posts or something else that will catch their eye so they go to another page.