Alexa is a name of a website ranking system set up by alexa.com, an Amazon company. The rank, often refer to as Alexa Rank, Alexa Rack Checker, or Alexa Traffic Rank, displays a website popularity over the past three months based on traffic and engagement data gathered from users who has installed the Alexa Toolbar web browser extension. The extension is available for Google Chrome, Firefox, and Internet Explorer and is available at www.alexa.com/toolbar
The data are estimated from user browsing activities and is analyzed and serve as a metric for determining a website's overall popularity in relation to other websites. With Alexa Rank, the lower the number the better. For example, a website that ranks "1" indicates it is most popular during the past three months. This rank, along with other website statistics, is often used by web masters and SEO experts to identify a site's position compared to that of the competitors, to enhance marketing potential for advertisers, and visitor engagement.
Regardless, Alexa Rank data can effective be used to gain a general estimation of a website's popularity and to help gain competitive intelligence of your website against that of your competitors.