What is a bounce rate?
The bounce rate represents the percentage of visitors that enter a site and then leave it without visiting the rest of the pages within that same site. It could represent an estimate of how interested are the users that you have impacted in the product you are advertising.
Bounce rates tend to vary a lot depending on the device you are serving your ad, the type of sites (retail, content, portals...), the audience, etc. so it's hard to estimate the average bounce percentage.
It's common in the industry to measure the bounce rate with Google Analytics (GA from now on). The definition of bounce rate in GA is slightly different than the industry standard:
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. Bounce rate is single-page sessions divided by all sessions or the percentage of all sessions on your site in which users viewed only a single page and triggered only a single request to the Analytics server.