Alexa Rank is a number which is generated based on the traffic of your website.

Alexa Ranking is a ranking system set by alexa.com that basically audits and makes public the frequency of visits on various web sites. It is based on the amount of traffic recorded from users that have the Alexa toolbar installed over a period of three months.
This traffic is based on such parameters as reach and page views. The reach refers to the number of Alexa users who visit a particular site in one day. Page view, is the number of times a particular page (URL) is viewed by Alexa users.