Alexa is a product of amazon. It have a alogo to check website rank. You will see many a websites also shows website worth they have some algo hope my next post have this part too.
Alexa rank manily deponds upon visitors of your website, but it also depends on many other things too.
This traffic is based on such parameters as reach and page views.
Alexa rank is also depends upon three other major parts like backlinks, speed of website, popularity of website and website up time also play major role in alexa rank.