Bounce rate is pretty simple. it is someone that lands on your site from an external source and then exits without travelling to another page on your site.

So for example if 1 person sees a PPC ad for amazon, follows it then don't like what they see and click the back button to look else where... that is one visit and 100% bounce rate. if that person clicks on the more info or one of the similar products then they travel to another page on Amazon resulting in 0% bounce rate.

Does that make sense?

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In simple language i come to your site and leave your site immediately.Its just one page visit, i mean landing and exit page are same.If bounce rate is very high it meands users are not getting what they are looking in your site or simply your site is not relevant to search query.