A bounced visitor is someone who leaves your site without interacting with it at all. The share of visitors who do this is your bounce rate.
On most shops, between 40% and 75% of visitors bounce. A bounce rate below 25% is rare, and we've only seen it on a few sites. Above 75%, it's time to start making changes.
Until 2012, there was no way to know who these visitors were, and once they left they were usually lost. Then Google introduced remarketing in its display advertising, which lets you show banners or ads to the people who left.
And it works. Think of a time you wanted to buy something online but couldn't decide straight away, or the phone rang before you finished the order. Even if you didn't bounce, the shop lost you. The next day you couldn't remember the site, or you had to put the purchase off. Remarketing brings the shop back in front of you.