Bounce Rate essentially represents a measure of website attractiveness. It measures the percentage of initial visitors to a website who 'bounce' away to a different site, rather than continue on to other pages within the same site.
This indicator will be included in my book: Key Performance Indicators - the 100 measures every manager needs to know, which contains an in-depth description of this KPI, as well as practical advice on data collection, calculations, target setting, and actual usage.