The length of time a visitor spends exploring a web site or web page is a measurement of that site or page's stickiness.  A sticky site is one that people spend more time on. Marketers generally want to create sticky sites, because the more time a visitor spends engaged on the site, the more likely they are to respond to the company's call to action.

In the Web 2.0 World (and beyond), the search engines will be factoring in site stickiness and time spent as part of their algorithms to determine the rank of sites.  It stands to reason that a site that is more sticky will be considered more relevant (based on a keyword query) and thus considered more authoritative by the search engines and the user.