What is the “Bounce Rate” in Google Analytics?
The number of times unique users returned to your website in a given time period
The percentage of visits when a user landed on your website and exited without any interactions
The percentage of sessions in which a user exits from your homepage
The percentage of total site exits
A bounce is a single-page session on your site. In Analytics, a bounce is calculated specifically as a session that triggers only a single request to the Analytics server, such as when a user opens a single page on your site and then exits without triggering any other requests to the Analytics server during that session.
Read more here: https://support.google.com/analytics/answer/1009409